RARY 

"RSiTY  OF 
IFORNIA 
'A  CRUZ 


• 

STUDIES  IN  LUMINESCENCE,^, 


BY 


EDWARD  L.  NICHOLS  and  ERNEST  MERRITT 

Professors  of  Physics  in  Cornell  University 


WASHINGTON,  D.  C. 

PUBLISHED  BY  THE  CARNEGIE  INSTITUTION  OF  WASHINGTON 
1912 


CARNEGIE  INSTITUTION  OF  WASHINGTON 
PUBLICATION  No.  152 


PRESS   OF   GIBSON    BROTHERS 
WASHINGTON,  D.  C. 


PREFACE. 

The  series  of  investigations  described  in  this  memoir  was  begun  in  1903. 

The  authors  believed  that  by  the  use  of  quantitative  methods  and  par- 
ticularly by  the  application  of  the  spectrophotometer  to  the  study  of  the 
spectra  of  fluorescent  and  phosphorescent  substances,  something  of  definite 
value  might  be  added  to  the  existing  information  concerning  luminescence. 

Spectroscopy,  whether  visual  or  photographic,  is  a  method  of  high 
precision  where  applied  to  line  spectra,  but  in  the  case  of  the  broad  bands 
of  the  spectra  of  fluorescent  solids  and  liquids  it  affords  but  little  informa- 
tion beyond  the  approximate  width  and  general  location  of  the  bands.  The 
spectrophotometer,  on  the  other  hand,  enables  the  observer  to  determine 
the  distribution  of  intensities  throughout  the  emission  bands  and  the  coeffi- 
cients of  absorption  for  the  various  wave-lengths  of  the  corresponding 
absorption  bands.  From  the  curves  expressing  the  results  of  such  measure- 
ments, moreover,  it  is  possible  to  locate  with  considerable  accuracy  the 
crests  of  the  bands.  One  may  thus  attain  some  detailed  knowledge  of  the 
laws  of  the  radiation  of  luminescence,  compare  luminescence  with  the  radi- 
ation due  to  temperature,  and  obtain  a  basis  for  theoretical  discussion. 

A  number  of  important  portions  of  the  work  described  in  this  volume  have 
been  carried  out  at  our  suggestion  by  Doctors  Frances  G.  Wick,  C.  A. 
Pierce,  Percy  Hodge,  and  C.  W.  Waggoner,  and  by  Messrs.  H.  E.  Howe 
and  Carl  Zeller.  To  these  investigators  and  also  to  Prof.  W.  R.  Orndorff, 
who  has  repeatedly  aided  us  by  undertaking  the  preparation  of  fluorescent 
compounds  and  by  suggestions  concerning  the  chemical  aspects  of  the 
problem,  we  desire  to  express  our  indebtedness. 

The  recent  exhaustive,  thorough,  and  discriminating  review  of  the  very 
large  literature  relating  to  luminescence  published  by  Professor  Kayser 
(H.  Kayser,  Handbuch  der  Spektroskopie,  Bd.  iv,  Kap.  v  and  vi)  in  his 
Handbook  of  Spectroscopy,  makes  any  extended  bibliographic  or  historic 
treatment  here  unnecessary  and  we  have  therefore  given  only  such  refer- 
ences to  previous  researches  as  bear  directly  upon  the  subjects  under 
consideration. 

The  subject-matter  contained  in  the  several  chapters  appeared  from 
time  to  time,  as  each  portion  of  the  work  reached  completion,  in  a  series 
of  papers  in  the  Physical  Review.  In  gathering  this  material  together  in  a 
single  treatise  we  have  recast  and  rearranged  it,  but  have  preserved  the 
original  form  of  presentation  in  so  far  as  it  was  found  to  be  consistent  with 
our  views  after  the  completion  of  the  work. 

Grants  which  were  received  from  the  Carnegie  Institution  of  Washington 
in  1905,  1909,  and  1910  have  greatly  facilitated  the  prosecution  of  the  experi- 
ments described  in  this  memoir  and  of  others  now  in  progress  and  have 
furthered  the  preparation  for  investigations  which  it  is  proposed  to  under- 
take in  the  near  future. 

PHYSICS  LABORATORY  OF  CORNELIA  UNIVERSITY, 

May  23,  igu. 

HI 


TABLE  OF  CONTENTS. 


PAGE 

Preface  ................................................................  in 

Chapter. 

I.  A  SPECTROPHOTOMETRIC  STUDY  OF  FLUORESCENCE  ....................  i 

Rhodamin  and  resorcin-blau  ..................................  12 

Quinine  sulphate  ..............................................  13 

Chlorophyll  ................................................  17 

Canary  glass  ...............................................  19 

Fluorite  ....................................................  20 


Summary  ....................................................  24 

II.  ON  THE  ABSORBING  POWER  AND  THE  FLUORESCENCE  OF  RESORUFIN.  .  .  25 

Historical  ..................................................  26 

Method  of  observation  .......................................  27 

Composition  of  resorufin  .....................................  27 

Absorption  and  thickness  ....................................  28 

Absorption  and  concentration  .....................  .  ..........  32 

Fluorescence  and  concentration  .....................  ..........  33 

Summary  ....................................................  38 

III.  THE  LUMINESCENCE  OF  SIDOT  BLENDE  .............................  41 

Luminescence  excited  by  Roentgen  rays  .......................  41 

Photo-luminescence  during  excitation  ..........................  42 

Failure  of  Stokes's  law  ......................................  45 

The  phosphorescence  spectrum  during  decay  ...................  45 

IV.  THE  DECAY  OF  PHOSPHORESCENCE  IN  SIDOT  BLENDE  AND  CERTAIN  OTHER 

SUBSTANCES  ..............................................  51 

Early  stages  in  the  decay  of  phosphorescence  in  Sidot  blende  ........  53 

The  study  of  phosphorescence  in  Sidot  blende  and  certain  other  sub- 

stances during  the  whole  period  of  decay  .....................  58 

Experimental  method  ........................................  58 

Experiments  with  Sidot  blende  ...............................  59 

Experiments  with  different  phosphorescent  substances  ...........  67 

Summary  ....................................................  69 

V.  THE  INFLUENCE  OF  THE  RED  AND  INFRA-RED  RAYS  UPON  THE  PHOTO- 

LUMINESCENCE  OF  SIDOT  BLENDE  ..........................  71 

Effect  of  the  longer  waves  before  excitation  ....................  71 

Influence  of  the  longer  waves  during  excitation  .................  73 

Effect  of  the  longer  waves  during  decay  ........................  76 

Variations  of  the  effect  with  the  length  of  the  longer  waves  .......  83 

VI.  VARIATIONS  IN  THE  DECAY  OF  PHOSPHORESCENCE  PRODUCED  BY  HEATING  .  .  85 

Experiments  with  Sidot  blende  ..................................  85 

Experiments  with  Balmain's  paint  .....................  ..........  96 

VII.  STUDIES  OF  PHOSPHORESCENCE  OF  SHORT  DURATION  .................  109 

Dr.  Waggoner's  studies  in  phosphorescence  of  short  duration  .......  109 

Methods  of  measurement  ....................................  1  09 

Methods  of  preparing  the  phosphorescent  compounds  ............  no 

Discussion  of  results  .........................................  114 

Effect  of  infra-red  on  the  initial  decay  of  Sidot  blende  ............  117 

Decay  curves  for  different  wave-lengths  ........................  1  1  8 

Spectrophotometric  study  of  the  cadmium-sodium  compounds.  ...  119 

Summary  ..................................................  121 

The  experiments  of  Mr.  Carl  Zeller  ..............................  121 

The  aniline  dyes  ............................................  122 

The  manganese-chloride  group  NaCl-MnCl2  ....................  122 

The  cadmium  group  .........................................  124 

Substances  of  slow  decay  .....................................  124 


VI  CONTENTS. 

Chapter.  PAGE 

VIII.  PHOTOGRAPHIC  STUDIES  OF  LUMINESCENCE 125 

The  distribution  of  energy  in  the  fluorescence    spectrum  and  the 

phosphorescence  spectrum  of  Sidot  blende 125 

Phosphorescence  at  room  temperature 128 

Discussion  of  method 129 

The  phosphorescence  spectrum  during  decay  and  the  question  of 

two  overlapping  bands 131 

Experimental 132 

A  photographic  test  of  the  effect  of  infra-red  rays 134 

The  effect  of  temperature  on  the  fluorescence  spectrum 135 

Discussion  of  results 136 

IX.  A  SPECTROPHOTOMETRIC  STUDY  OF  CERTAIN  CASES  OF  KATHODO-LUMI- 

NESCENCE 137 

Willemite 140 

Sidot  blende 141 

Dependence  of  kathodo-luminescence  upon  current  and  discharge 

potential 143 

Conclusions 1 44 

X.  ON  THE  ELECTRICAL,  PROPERTIES  OF  FLUORESCENT  SOLUTIONS  AND  VAPORS  147 

The  influence  of  fluorescence  upon  electrical  conductivity 147 

Photo-active  cells  with  fluorescent  electrolytes  (Dr.  Hodge's  experi- 
ments)    152 

Mr.  Howe's  experiments  on  fluorescent  anthracene  vapor 1 63 

Apparatus 164 

Observations  and  results 165 

Conclusions 166 

XI.  ON  FLUORESCENCE  ABSORPTION 167 

XII.  THE  DISTRIBUTION  OF  ENERGY  IN  FLUORESCENCE  SPECTRA 175 

Determination  of  the  distribution  of  energy  in  the  spectrum  of  the 

comparison  flame 1 75 

Comparison  of  the  fluorescence  spectra  with  the  spectrum  of  the 

standard  acetylene  flame 179 

The  correction  for  slit  width 182 

The  correction  for  absorption 1 85 

The  energy  curves  of  fluorescence 1 86 

XIII.  THE  SPECIFIC  EXCITING  POWER  OF  THE  DIFFERENT  WAVE-LENGTHS  OF 

THE  VISIBLE  SPECTRUM  IN  THE  CASE  OF  THE  FLUORESCENCE  OF 

EOSIN  AND  RESORUFIN 187 

Coefficients  of  absorption 189 

Computation  of  specific  exciting  power 19" 

XIV.  THE  THEORY  OF  WIEDEMANN  AND  SCHMIDT 195 

XV.  THE  PHENOMENA  OF  PHOSPHORESCENCE  CONSIDERED  FROM  THE  STAND- 
POINT OF  THE  DISSOCIATION  THEORY 201 

Summary  of  experimental  laws 201 

The  decay  of  phosphorescence  under  simple  conditions 202 

Absorption  effects 205 

Influence  of  irregularities  in  distribution  of  the  active  material 208 

Diffusion  effects 211 

Influence  of  ionic  grouping 212 

Hysteresis,  temperature  effects,  etc.,  explained  by  ionic  grouping.  .  218 

Summary 222 

INDEX 224 


STUDIES  IN  LUMINESCENCE 


BY 


EDWARD  L.  NICHOLS  and  ERNEST  MERRITT 

Professors  of  Physics  in  Cornell  University 


CHAPTER  I. 
A  SPECTROPHOTOMETRIC  STUDY  OF  FLUORESCENCE.1 

The  law  expressed  by  Stokes2  in  his  memoir  entitled  "The  Change 
of  Refrangibility  of  Light,"  to  the  effect  that  in  fluorescence  the  fluorescent 
light  is  always  of  greater  wave-length  than  the  exciting  light,  has  been  called 
in  question  by  Lommel,  who  pointed  out  that  for  certain  fluorescent  bodies 
there  is  an  unmistakable  overlapping  of  the  regions  in  the  spectrum  occupied 
by  the  exciting  light  and  by  the  fluorescence  which  it  produces.  Lommel 
made  the  further  very  important  statement  that  for  this  class  of  substances 
the  character  and  composition  of  the  fluorescence  spectrum  are  independent 
of  the  wave-length  of  the  exciting  light.  Lommel's  results,  in  so  far  as 
they  had  to  do  with  the  non-validity  of  Stokes's  law,  were  confirmed  by 
Hagenbach.3  A  few  years  later  Lubarsch4  published  measurements  in 
confirmation  of  Stokes's  law.  A  later  paper  by  Lommel,5  in  which  he 
described  the  fluorescence  of  the  so-called  chameleon  colors,  led  Hagenbach 
to  new  experiments,  in  the  course  of  which  he  discovered  what  he  believed 
to  be  a  source  of  error  in  his  former  measurements,  and  he  reaffirmed  the 
law  of  Stokes  for  all  such  substances.  In  1877,  Brauner6  obtained  results 
in  confirmation  of  Lommers  view.  In  1879,  Lubarsch7  published  further 
experiments  on  fluorescence,  this  time  in  favor  of  Lommel's  results.  Laman- 
sky8  in  1879  described  measurements  in  confirmation  of  Stokes's  law.  In 
a  still  later  paper  Hagenbach9  returned  to  the  defense  of  Stokes's  law  as 
against  Lommel10  and  Lubarsch,11  who  in  the  meantime  had  published 
further  articles  dealing  with  his  objections  and  criticizing  Lamansky's 
method.  Wesendonck12  in  1885  made  observations  with  the  sun's  spectrum, 
using  two  concave  mirrors  and  a  prism,  in  the  course  of  which  he  obtained 
conclusive  evidence  that  the  fluorescence  of  naphthalin-roth  extended  to 
wave-lengths  shorter  than  that  of  the  exciting  light.  In  1886,  Stenger13 
took  the  question  up  at  length.  He  found  that  whether  he  used  Hagen- 
bach's  method  of  illuminating  the  free  surface,  Lommel's  method  of  grazing 
incidence  through  the  side  of  a  flask,  or  Lubarsch's  fluorescent  eye-piece, 
his  measurements  confirmed  Lommel  as  to  the  invalidity  of  Stokes's  law 
but  not  as  to  the  independence  of  the  fluorescent  spectrum  from  the  char- 
acter of  the  exciting  light.  He  also  made  experiments  in  collaboration  with 
Hagenbach,  who  was  finally  converted  to  the  same  view. 

It  is  our  purpose  in  this  chapter  to  describe  results  obtained  by  the 
application  of  the  spectrophotometer  to  the  measurement  of  the  fluo- 
rescence spectrum  of  those  substances  concerning  the  fluorescence  of 


'Physical  Review,  xvnr,  p.  403,  and  xix,  p.  18. 

'Stokes,  Phil.  Trans.,  p.  463,  1852. 

'Hagenbach,  Poggendorff's  Ann.,  146,  pp.  65,  232,  373,  508. 

'Lubarsch,  Poggendorff's  Ann.,  153,  p.  428,  1874. 

'Lommel,  Poggendorff's  Ann.,  159,  p.  514,  1876. 

'Brauner,  Wiener  Anzeiger,  19,  p.  178,  1877. 

"Lubarsch,  Wied.  Ann.,  6,  p.  248. 

•Lamansky,  Comptes  Rendus,  88,  p.  1192,  1879. 

'Hagenbach,  Wied.  Ann.,  18,  p.  45. 

10Lommel,  Wied.  Ann.,  8,  p.  244,  1879. 

"Lubarsch,  Wied.  Ann.,  9,  p.  665,  1980;  also  Wied.  Ann.,  n,  p.  68,  1880. 

12Wesendonck,  Wied.  Ann.,  26,  p.  521,  1885. 

"Stenger,  Wied.  Ann.,  28,  p.  201. 


STUDIES   IN  LUMINESCENCE. 


which,  especially  with  reference  to  the  validity  of  Stokes 's  law,  the  long- 
continued  discussion  already  described  arose.  But  few  attempts  have 
been  made  to  apply  the  spectrophotometer  to  the  study  of  fluorescence; 
yet  it  is  obviously  possible  to  determine  both  the  limits  and  the  maximum 
of  a  spectral  region  for  which  a  curve  of  intensities  can  be  plotted  with 
far  greater  accuracy  than  by  the  method  hitherto  pursued  by  all  observers, 
*.  e.,  that  of  attempting  to  set  the  cross-hair  in  the  eye-piece  of  a  spectro- 
scope in  the  region  of  greatest  brightness,  or  at  the  point  where  the  spectrum 
ceases  to  be  visible.  Experimenters  have  perhaps  been  deterred  from  the 
use  of  the  spectrophotometer  because  of  the  faintnessof  fluorescence  spectra. 
It  is  true  that  the  fluorescent  light  from  many  substances  is  so  weak  as  to 
preclude  all  measurements  of  its  spectrum;  but  it  is  also  true,  as  we  have 
found  in  the  course  of  the  experiments  to  be  described,  that  settings  can 
be  made  in  cases  where  the  brightness  of  the  spectrum  is  far  below  that 
necessary  to  arouse  the  sense  of  color  and  where  the  presence  of  light  can 
be  detected  only  after  prolonged  shielding  of  the  eye.  The  use  of  the 


o 


A 


Fig.  i. 

cross-hair  in  such  cases  is  out  of  the  question,  for  the  field  is  much  too  dim 
to  render  it  visible,  while  every  attempt  to  illuminate  it  from  the  side  would 
flood  the  eye-piece  with  light  sufficient  to  quench  that  under  observation. 

The  instrument  used  in  most  of  our  observations  was  the  spectropho- 
tometer of  Lummer  and  Brodhun.  In  many  of  our  earlier  measurements 
the  ocular  lenses  in  the  eye-piece  were  used,  the  eye  being  focused  upon  the 
aperture  in  the  eye-piece  and  not  upon  the  face  of  the  prism.  By  means  of 
metal  screens  attached  to  the  collimator  slits  of  the  instrument  the  length  of 
slit  was  regulated  so  as  to  avoid  overlapping  of  the  spectral  images  and  to 
give  two  contiguous  spectra  in  the  field  of  view.  One  loses  in  this  way  the 
advantage  of  the  method  of  contrast,  but  the  brightness  of  the  field  is 
greatly  increased.  Later  on  the  contrast  field  was  employed  because  it  was 
found  to  be  less  fatiguing  to  the  eye  and  on  the  whole  more  accurate.  Since 
in  most  cases  it  was  desired  to  employ  monochromatic  light  for  the  excita- 
tion of  fluorescence,  the  spectrophotometer  was  employed  in  connection 
with  a  large  spectrometer,  as  shown  in  Fig.  i .  The  eye-piece  and  slit  of  the 


A   SPECTROPHOTOMETRIC   STUDY   OF   FLUORESCENCE. 


EXCITING 
LIGHT'-' 


FLUORESCENT 
SOLUTION 


spectrometer  were  removed,  and  in  place  of  the  latter  a  Nernst  filament  (N) 
was  mounted  vertically.  This  filament  afforded  a  nearly  linear  source  of 
light,  giving  a  sufficiently  powerful  continuous  spectrum.  The  filament  was 
attached  to  the  arm  carrying  the  collimator  tube  so  as  to  move  with  the 
latter  and  to  remain  in  the  vertical  plane  passing  through  its  axis.  The 
observing  telescope  was  clamped,  and  different  portions  of  the  spectrum 
were  brought  into  the  field  as  desired  by  movements  of  the  collimator  tube. 
The  liquid,  the  fluorescence  of  which  was  to  be  studied,  was  placed  in  a  rec- 
tangular cell  (C) ,  upon  one  face  of  which  a  real  image  of  the  spectrum  was 
focused.  This  face  of  the  cell  was  provided  with  a  metal  screen  having  a 
vertical  opening  i  mm.  wide,  through  which  the  light  used  for  exciting  fluo- 
rescence could  pass.  This  opening  (see  Fig.  2)  was  so  placed  that  the  exciting 
light  entering  the  cell  would  be  parallel  to  the  adjacent  face  of  the  cell  and  as 
near  to  the  same  as  practicable.  In  adjusting  the  metal  screen  one  edge 
of  the  slit  or  opening  was  made  to  exactly  cover  the  glass  forming  this  face 
of  the  cell,  as  shown  in  the  figure.  The  fluorescence  spectrum  was  observed 
from  a  direction  at  right  angles 
to  the  path  of  the  exciting  beam,  | 

and  in  order  to  bring  the  brightest 
fluorescence  regions  of  the  liquid 
into  the  field  the  vertical  plane  of 
the  collimator  of  the  spectropho- 
tometer  was  ad  justed  so  as  to  bring 
into  the  field  of  view  the  layer  of 
liquid  lying  next  to  the  face  of  the 
cell  through  which  the  exciting 
light  entered.  By  this  arrange-  * 

ment  the  width  of  the  opening  g 

through  which  the  light  entered 
the  cell  and  the  width  of  the  slit  o3 

3 

of    the   spectrophotometer    each  i 

being  i  mm.,  the  average  depth  pig.  2. 

of  liquid  within    which  fluores- 
cence was  produced  was  approximately  0.55  mm.  and  the  average  distance 
which  the  fluorescence  light  passed  through  the  liquid  before  leaving  the 
cell  was  also  0.55  mm. 

The  source,  A,  of  the  comparison  spectrum  (Fig.  i)  was  an  acetylene 
flame,  the  light  from  which  was  reflected  diffusely  from  the  face  of  the 
block  of  magnesium  carbonate  (M)  mounted  at  an  angle  of  45°  at  the 
end  of  the  collimator  slit.  Measurements  were  made  by  varying  the  width 
of  the  slit  until  the  two  regions  of  the  spectrum  under  observation  were 
equally  bright. 

The  substances  specified  by  I^ommel  as  belonging  to  his  first  class,  in 
which  it  is  possible  to  excite  fluorescence  by  means  of  light  of  wave-length 
longer  than  that  of  a  portion  of  the  fluorescence  spectrum,  and  in  which 
the  distribution  of  intensities  in  the  fluorescence  spectrum  is  independent 
of  the  character  of  the  exciting  light,  are  naphthalin-roth,  eosin,  and 
chlorophyll.  To  this  list  Stenger  added  the  substance  fluorescein.  The  last- 
named  substance,  on  account  of  its  intense  fluorescence  and  the  location 


AGLASS 


4  STUDIES  IN   LUMINESCENCE. 

of  the  fluorescent  band  in  the  middle  of  the  spectrum,  in  the  regions  of  the 
highest  luminosity,  was  selected  for  detailed  study  for  the  purpose  of  test- 
ing the  conclusions  reached  by  Lommel  and  the  other  investigators  men- 
tioned in  the  opening  paragraphs  of  this  chapter. 

Ten  cubic  centimeters  of  alcohol  were  saturated  with  fluorescein  at  room 
temperature  and  the  solution  was  filtered.  To  40  c.  c.  of  distilled  water 
one  drop  of  a  normal  solution  of  sodium  carbonate  (Na2CO3)  was  added. 
Two  parts  of  the  concentrated  alcoholic  solution  were  then  mixed  with  100 
parts  of  the  water  thus  rendered  alkaline.  The  fluorescence  spectrum  of 
this  solution,  excited  by  the  undispersed  rays  of  the  acetylene  flame,  was 
first  measured,  the  distribution  of  intensities  being  compared,  as  in  nearly 
all  our  subsequent  experiments,  with  the  spectrum  of  the  light  diffusely 
reflected  from  the  surface  of  the  block  of  magnesium  carbonate  shown  in 
Fig.  i .  The  absorption  spectrum  of  the  solution  was  then  taken,  the  trans- 
mission through  the  cell,  which  had  a  thickness  of  i.i  cm.,  being  meas- 
ured by  means  of  the  spectrophotometer.  The  source  of  the  transmitted 
light  was  a  second  similar  block  of  magnesium  carbonate  illuminated  by 
the  same  acetylene  flame  that  served  for  the  comparison  spectrum. 

Much  stress  having  been  laid  by  some  of  the  previous  observers  upon 
the  influence  of  stray  light,  the  following  measurements  were  made.  The 
cell  was  filled  with  distilled  water,  set  up  precisely  in  the  position  in  which 
it  had  been  placed  in  the  study  of  the  fluorescence  spectrum,  and  similarly 
illuminated  by  means  of  the  acetylene  flame.  No  measurable  stray  light 
was  found,  but  an  exceedingly  weak  fluorescence  spectrum  due  to  the  glass 
walls  of  the  cell  was  detected.  Since  the  maximum  of  the  fluorescence 
spectrum  of  the  glass  was  found  to  lie  further  to  the  violet  than  the  fluores- 
cence spectrum  of  the  fluorescein,  and  since  moreover  it  was  of  scarcely 
measurable  intensity,  it  was  not  deemed  necessary  to  take  further  cog- 
nizance of  these  sources  of  error. 

To  determine  the  fluorescence  spectrum  of  the  solution  when  excited 
by  monochromatic  light,  a  mercury  arc-light  of  the  Lummer  pattern  was 
used  for  excitation.  It  was  found  that  the  violet  lines  from  the  spectrum  of 
this  arc  produced  fluorescence  corresponding,  as  regards  the  position  of  the 
maximum  and  the  general  form,  with  the  curve  previously  obtained  by 
means  of  the  light  of  the  acetylene  flame,  but  that  the  green  line  (X  =  o.575) 
excited  no  fluorescence.  This  latter  result  was  to  be  expected,  since  this 
line  lies  altogether  outside  of  the  absorption  band  of  the  solution  in  a  region 
for  which  almost  complete  transparency  exists.  The  spectrum  of  the  acety- 
lene flame  was  subsequently  tried  as  an  exciting  source,  but  it  was  too  weak 
to  give  easily  measurable  intensities  of  fluorescence.  The  slit  of  the  spec- 
trometer was  then  removed  and  a  Nernst  filament,  N,  was  mounted  in  the 
axis  of  the  collimator  tube  as  shown  in  Fig.  i .  This  filament  was  found  to 
be  of  abundant  brilliancy.  It  was  maintained  at  constant  brightness  by 
means  of  a  variable  resistance,  which  was  adjusted  whenever  the  fluctua- 
tions of  an  ammeter  placed  in  the  electric  circuit  indicated  it  to  be  necessary. 

Measurements  of  the  fluorescence  spectrum  of  the  solution  were  made 
by  means  of  the  spectrum  of  the  Nernst  filament,  using  as  exciting  light 
three  nearly  monochromatic  regions  of  wave-length,  X=  0.518  n  to  0.536  n, 
X=  0.487  n  to  0.502  ju  and  X=  0.460/1  to  0.471  /*.  The  curves  thus  obtained 


A   SPECTROPHOTOMETRIC    STUDY   OF    FLUORESCENCE. 


are  plotted  in  Fig.  3,  together  with  the  curve  of  transmission  for  the  solu- 
tion. It  will  be  seen  from  this  figure  that  the  maximum  of  intensity  of  the 
fluorescence  spectrum  in  these  three  cases  lies  in  the  same  region,  at  0.517/1, 
and  that  there  is  no  evidence  of  any  shifting  of  the  fluorescence  spectrum 
with  the  wave-length  of  the  exciting  light.  It  is  obvious,  moreover,  that 
not  only  is  it  possible  in  the  case  of  this  solution  to  obtain  fluorescence  of 
refrangibility  greater  than  that  of  the  exciting  light,  but  that  in  the  case 
of  the  curve  marked  A  the  maximum  of  the  fluorescence  spectrum  is  of 
shorter  wave-length  than  the  shortest  wave-length  used  in  excitation. 
These  curves  likewise  agree  fully  in  character,  and  as  regards  the  position 
of  their  maximum,  with  that  for  the  fluorescence  spectrum  of  the  same 
solution  when  excited  by  the  un- 
dispersed  light  of  the  acetylene 
flame.  (Not  shown  in  the  figure.) 
These  curves  for  the  fluorescence 
spectrum  do  not  correspond  pre- 
cisely with  the  typical  curve,  mean- 
ing by  that  term  the  curve  repre- 
senting the  distribution  of  intensi- 
ties in  the  fluorescence  spectrum 
of  the  surface  layer  of  the  fluores- 
cing  liquid.  Itis  possible,  however, 
in  the  case  of  a  non-turbid  medium, 
to  compute  from  the  observed 
curve  the  approximate  form  of  the 
typical  curve.  Fig.  4  shows  graph- 
ically the  result  of  such  a  compu- 
tation.1 Curve  A  gives  the  trans- 
mission of  a  glass  cell  containing 
a  layer  i.i  cm.  in  thickness  of  the 
fluorescein  solution.  The  dotted 

curve  of  similar    form    gives  the  Fig-  3- — Fluorescein. 

transmission  corrected  for  losses 
in  the  cell  when  filled  with  distilled 
water.  From  this,  by  the  well- 
known  law  of  variation  of  absorption 

with  the  thickness,  the  curve  D  is  found  for  a  layer  0.055  cm-  in  thickness, 
which  is  the  estimated  mean  distance  through  the  solution  which  the 
fluorescent  light  passes  before  entering  the  slit  of  the  spectrophotometer. 
The  curve  B  is  the  observed  curve  of  the  distribution  of  the  intensities  in 
the  fluorescence  spectrum,  and  from  this  was  computed  the  curve  E,  which 
represents,  as  nearly  as  the  accuracy  of  the  data  will  allow,  the  typical  curve 
for  this  substance,  corrected  for  absorption.  It  will  be  seen  that  in  the 
case  of  this  solution,  under  the  conditions  of  the  measurements,  the  absorp- 
tion of  the  fluorescent  light  by  the  solution  produces  only  a  slight  shift  of 
the  maximum  toward  the  longer  wave-lengths.  When  the  fluorescent  light 
passes  through  a  considerable  layer  of  the  solution  the  effect  of  absorption 
is  much  more  marked  and  there  is  a  decided  change  of  color.  When  a  thin 

'In  this  figure  and  in  Figs.  5,  6,  7,  and  8  the  scale  of  wave-lengths  has  been  doubled. 


40 


20 


Fluorescence  spectra  obtained  when  the  exciting  light 
lies  in  different  regions  of  the  spectrum.  For 
curve  A,  the  exciting  light  was  confined  to  the 
region  marked  A  on  the  horizontal  axis,  etc. 
Vertical  scales  arbitrary. 


STUDIES   IN   LUMINESCENCE. 


layer  of  the  solution  of  fluorescein  is  viewed  by  reflected  light  its  color  is 
green,  whereas  the  fluorescence  of  a  mass  of  the  liquid  appears  decidedly 
yellowish.  This  change  is  shown  graphically  in  the  curves  of  Fig.  5.  In 


\ 


\ 


ICO 


.500/^  2O  40 

Fig.  4.  —  Fluorescein. 

Typical  fluorescence  spectrum  (EE). 


this  figure  A  is  the  transmission  curve  of  the  solution;  B  is  the  observed 
curve  of  fluorescence  when  the  slit  through  which  the  excited  light  enters  the 
cell  is  placed  so  that  the  fluorescent  light  passes  through  0.055  ]cm.  ofjlie 


.450/J-    60        70        80        90     .5CO/<      1O        2O        3O       40       50        60       70 

Fig.  5. — Fluorescein. 

Effect  of  absorption  upon  the  fluorescence  spectrum. 

solution  before  exit;  and  C  is  the  fluorescence  curve  when  the  slit  is  shifted 
to  such  a  position  that  the  fluorescent  light  passes  through  i  cm.  of  the  solu- 


A   SPECTROPHOTOMETRIC  STUDY  OF  FLUORESCENCE.  7 

tion.  It  will  be  seen  that  the  maximum  is  shifted  from  0.516  /z  to  0.522  /z, 
and  that  while  the  two  curves  are  nearly  coincident  on  the  side  toward  the 
red  the  values  on  the  other  side  of  the  curve  fall  off  very  rapidly  as  the 
result  of  the  increased  absorption.  The  boundary  of  the  fluorescence  spec- 
trum toward  the  violet  in  the  one  case  would  lie  at  about  wave-length  0.505  /z, 
whereas  in  the  thinner  layer  it  would  be  visible  to  at  least  0.490  /*.  It  is 
obvious  that  the  color  of  the  fluorescence  in  the  latter  case  will  contain  a 
great  excess  of  green. 

The  effect  of  diluting  the  fluorescent  solution  is  similar  to  that  of  diminish- 
ing the  distance  through  which  the  light  passes.  The  results  of  observa- 
tion upon  a  solution  of  fluorescein,  diluted  until  the  intensity  of  the  fluores- 
cence spectrum  was  diminished  as  far  as  would  permit  of  satisfactory 
readings,  are  shown  in  Fig.  6.  The  curve  A  A  represents  the  transmission 
of  the  cell  filled  with  the  dilute  solution  and  BB  is  the  distribution  curve 
of  its  fluorescence  spectrum.  The  dotted  lines  show  the  corresponding 


Fig.  6. — Fluorescein. 

Effect  of  dilution  upon  the  fluorescence  spectrum. 

transmission  curve  and  a  portion  of  the  fluorescence  curve  for  the  solution 
before  dilution.  It  will  be  noted  that  in  this  case,  as  in  the  case  of  the  com- 
parison of  thick  and  thin  layers  of  a  given  solution,  the  curves  are  coin- 
cident toward  the  red,  but  that  the  dilute  solution  has  its  maximum  shifted 
toward  the  green,  also  that  the  ordinates  on  this  side  of  the  curve  show 
an  increase  indicative  of  the  change  of  color,  which,  as  is  well  known,  is 
always  observed  as  the  result  of  diluting  the  fluorescent  solutions  of  this 
substance. 

Although,  as  has  been  shown  in  Figs.  4  and  5,  the  modifications  pro- 
duced by  absorption  in  the  curves  of  the  fluorescence  spectrum  may  be 
very  marked,  the  effect  of  absorption  diminishes  rapidly  with  dilution  of 
the  solution.  For  example,  if  we  apply  the  correction  for  absorption  to 
the  curve  for  the  dilute  solution  in  Fig.  6  we  find,  as  is  indicated  in  Fig.  7, 
that  the  change  is  insignificant.  In  this  figure  C  is  the  observed  curve  for 


8 


STUDIES  IN  LUMINESCENCE. 


the  fluorescence  of  the  dilute  solution,  A  the  transmission  curve  of  a  layer 
i.i  cm.  in  thickness,  and  B  the  computed  curve  for  the  transmission  of  the 
mean  layer  of  liquid  through  which  the  fluorescent  light  has  to  pass.  The 
correction  for  this  absorption  is  indicated  by  means  of  the  dotted  line. 

It  having  been  established  that  the  fluorescence  of  bodies  of  this  class  is 
independent,  as  regards  the  distribution  of  intensities,  of  the  wave-length 
of  the  exciting  source,  it  would  be  of  interest  to  inquire  whether  the  fluo- 
rescent energy  for  a  given  wave-length  of  the  fluorescence  spectrum  varies 
with  the  wave-length  of  the  exciting  light  when  the  energy  of  the  latter 
is  constant,  or  whether  it  depends  only  upon  the  energy.  The  rigorous 
determination  of  these  relations  involves  a  knowledge  of  the  distribution  of 
energy  in  the  spectrum  of  the  exciting  source,  a  difficult  matter  to  determine 
with  accuracy  for  the  shorter  wave-lengths  of  the  visible  spectrum.  The 
curve  which  is  shown  in  Fig.  8  may,  however,  be  of  some  interest  in  this 


100 


80 


.500,"  20  40 

Fig.  7. — Fluorescein. 

Typical  fluorescence  spectrum  for  dilute  solution. 

connection.  It  represents  the  intensity  of  fluorescence,  taken  at  the  maxi- 
mum of  the  fluorescence  spectrum  of  the  solution  of  fluorescein,  as  a  func- 
tion of  the  wave-length  of  the  exciting  light.  Curve  A  was  taken  with 
the  Nernst  filament  as  a  source,  curve  B  with  the  acetylene  flame.  The 
dotted  line  shows  the  absorption  band  for  a  layer  of  the  solution  i.i  cm. 
thick.  It  will  be  seen  that  fluorescence  begins  approximately  at  the  wave- 
length at  which  the  solution  begins  to  absorb,  and  that  the  maximum  lies 
well  within  the  absorption  band  but  is  shifted  to  the  red.  The  longer  wave- 
lengths within  the  band  are  more  effective  on  account  of  their  greater  energy. 
The  difference  in  the  form  of  the  curves  A  and  B  is  probably  ascribable  to 
the  different  distributions  of  energy  in  the  spectra  of  the  sources  of  light 
employed.  The  important  questions  of  the  actual  distribution  of  energy 
in  fluorescence  spectra  and  of  the  relative  effectiveness  of  equal  quantities 
of  energy  when  the  wave-length  is  varied,  in  producing  excitation,  are  con- 
sidered at  length  in  Chapters  XII  and  XIII  of  this  memoir. 


A   SPECTROPHOTOMETRIC    STUDY   OF   FLUORESCENCE. 


In  addition  to  the  measurements  on  fluorescein  the  fluorescence  spectra 
of  solutions  of  eosin  and  naphthalin-roth  were  also  studied  by  the  method 
already  described,  and  the  transmission  curves  of  the  solutions  were  taken. 


30 


60 


40 


20 


A30/U    40       50         60        70         80       90      .500/M    10       20        30        40 

Fig.  8. — Fluorescein. 

Intensity  of  fluorescence  as  a  function  of  wave-length  of  exciting  light.  Ordi- 
nates  give  the  intensity  of  luminescence  and  abscissas  the  wave-length  of  the 
exciting  light.  A  refers  to  Nernst  glower  as  source,  B  to  acetylene  flame. 

Dilute  solutions  in  alcohol  were  made,  that  of  the  naphthalin-roth  being 
about  ./^o  saturated.  The  results  obtained  with  these  solutions,  which  are 
shown  in  Figs.  9  and  10,  afford  additional  corroboration  of  the  statements 


100 


.6/4 


Fig.  9. — Eosin. 

Fluorescence  spectra  observed  when  the  exciting  light  lies  in  different 
regions  of  the  spectrum.  Curve  -4  was  obtained  when  the  excit- 
ing light  was  confined  to  the  region  marked  A  on  the  axis  of  wave- 
lengths, etc.  Vertical  scales  arbitrary. 


10 


STUDIES  IN   LUMINESCENCE. 


of  Lommel.  They  are  indeed  in  every  respect  analogous  to  those  obtained 
with  fluorescein  and  lead  to  the  same  conclusions.  Each  solution  was 
excited  by  three  distinct  regions  of  the  spectrum,  one  lying  as  far  toward  the 
red  as  practicable,  one  toward  the  blue,  and  one  at  an  intermediate  wave- 
length. Curves  showing  the  distribution  of  intensities  in  the  three  spectra 
thus  produced  are  shown  in  the  figures  and  it  will  be  noted  that,  as  in  the 
corresponding  curves  for  fluorescein,  the  position  of  the  maximum  is  entirely 
independent  of  the  wave-length  of  the  exciting  light  and  that  the  general 
character  of  the  curve  remains  unchanged.  In  the  case  of  these  two  solu- 
tions, as  in  that  of  fluorescein,  it  was  possible  to  obtain  a  measurable  amount 
of  fluorescence  by  the  use  of  light  of  wave-length  greater  than  that  of  the 
maximum  of  the  fluorescence  spectrum.  The  form  of  fluorescence  curve  is 
very  similar  for  these  three  substances,  but  each  has  its  own  place  in  the 


20 


Fig.  10. — Naphthalin-roth. 


Fluorescence  spectra  observed  when  the  exciting  light  lies  in  different  regions 
of  the  spectrum.  Curve  A  was  obtained  when  the  exciting  light  was 
confined  to  the  region  marked  A,  etc.  Vertical  scales  arbitrary. 

spectrum.  The  maximum  for  fluorescein  (Fig.  3)  is  at  0.5 1 7  n,  that  for  eosin 
at  0.580^,  and  that  for  naphthalin-roth  at  0.594  ju-  The  position  of  the  max- 
imum of  these  three  curves  with  reference  to  the  absorption  band  appears 
to  vary  with  the  different  substances.  The  maximum  for  eosin  coincides 
approximately  with  the  infra  edge  of  the  absorption  band;  that  for  fluo- 
rescein lies  slightly  (about  0.05  /z)  toward  the  violet,  while  the  maximum  for 
naphthalin-roth  is  much  farther  displaced  toward  the  short  wave-lengths. 

Lommel's  contention  that  it  is  possible  to  excite  fluorescence  in  eosin 
by  means  of  the  light  of  the  sodium  flame  is  fully  confirmed  by  the  data 
plotted  in  Fig.  9,  from  which  it  will  be  seen  that  the  exciting  light  by  means 
of  which  curve  A  was  obtained  had  a  mean  wave-length  almost  precisely 
equal  to  that  of  the  sodium  lines.  Since  it  was  found  possible  by  means  of 
light,  all  of  which  was  of  greater  wave-length  than  the  maximum  of  the 


A  SPECTROPHOTOMETRIC  STUDY  OF  FLUORESCENCE.  II 

I 

fluorescence  spectrum,  to  produce  fluorescence  of  sufficient  strength  for 
measurement  with  the  spectrophotometer,  it  follows  that  observable  fluo- 
rescence can  be  produced  by  light  of  even  greater  wave-length  than  that 
recorded  in  our  diagram. 

We  deem  the  evidence  already  given  in  the  foregoing  paragraphs  to  be 
conclusive,  so  far  as  these  substances  are  concerned;  but  in  view  of  the 
differences  of  opinion  among  physicists  as  regards  the  validity  of  Stokes's 
law  we  venture  to  add  the  following  description  of  a  determination  of  the 
wave-length  of  the  least  refrangible  monochromatic  light  which  we  found 
capable  of  exciting  fluorescence  in  the  three  solutions  with  which  this 
chapter  deals. 

To  insure  freedom  from  the  existence  of  possible  errors  due  to  stray 
light,  two  different  methods  were  employed  of  avoiding  it.  In  the  first 
the  exciting  light,  before  dispersion,  was  passed  through  a  solution  of  the 
substance  to  be  examined,  thus  filtering  out  those  rays  particularly  active 
in  producing  fluorescence.  The  filtered  light  was  then  dispersed  by  means 
of  the  large  spectroscope  already  described,  and  a  second  solution  was 
subjected  to  an  isolated,  nearly  monochromatic,  region  of  the  spectrum. 
The  wave-length  of  this  region  was  increased  until  the  last  trace  of  fluo- 
rescence, observed  directly  with  the  eye, 

was  about  to  disappear.     To  distinguish  TABLE  i . 

between  fluorescence  and  the  presence 

i-  i-rr  f  Maximum 

of  light   diffused  from  small  particles,  wave-length 

,,,.,.  .  ,  .          ,.         o  Substance.  exciting 

the  light  was  viewed  at  an  angle  of  90  observable 

through  a  Nicol  prism,  by  which  means 


Fluorescein i      o .  542 

Eosin !      0.589 

Naphthalin-roth j      o .  632 


diffuse  light  was  completely  excluded. 

The  limit  of  excitation  thus  determined 

lay,  as  had  been  anticipated,  farther  to 

the  red  than  in  the  cases  where  a  spectro- 

photometrically   measurable  fluorescence 

had  been  obtained,  excepting  in  the  case  of  eosin,  where  it  was  found  to 

coincide  almost  exactly  with  the  ultra  edge  of  the  band  used  in  exciting 

the  spectrum  shown  in  curve  A  (Fig.  9). 

The  second  method  consisted  in  sending  the  isolated  region  of  the 
dispersed  light  to  be  used  for  excitation  through  a  second  spectroscope  and 
determining  as  before  the  limit  of  excitability.  This  method  of  removing 
stray  light  has  been  extensively  used  and  is  well  known  to  be  effectual. 
The  source  of  light  in  both  cases  was  an  electric  arc. 

The  results  obtained  by  these  two  methods  were  identical  and  are  shown 
in  Table  i. 

These  wave-lengths,  all  of  which  lie  far  to  the  red  from  the  maximum 
of  the  fluorescence  spectrum,  are  indicated  in  Figs.  3,  8,  and  9  by  means 
of  the  vertical  lines  marked  I. 

The  results  obtained  with  solutions  of  fluorescein,  eosin,  and  naphthalin- 
roth  thus  fully  confirm  the  contention  of  Lommel1  that  in  the  case  of  these 
substances  Stokes's  law  is  not  fulfilled.  In  the  opinion  of  Lommel  all  other 
fluorescent  substances,  with  the  probable  exception  of  chlorophyll,  conform 
to  Stokes's  law,  the  shortest  wave-length  of  the  fluorescence  spectrum  being 

1Ix>mmel,  Poggendorff's  Ann.,  143,  159,  and  160. 


12 


STUDIES  IN  LUMINESCENCE. 


always  of  less  refrangibility  than  the  longest  wave-length  capable  of  exciting 
fluorescence.  We  later  extended  our  measurements  to  various  other  fluo- 
rescent substances,  determining  in  each  case  the  location  and  character  of 
the  absorption  band  with  which  fluorescence  is  associated  and  the  form  of 
the  curve  of  relative  intensities  in  the  fluorescence  spectrum. 

RHODAMIN   AND  RESORCIN-BLAU. 

Of  the  substances  examined  rhodamin  and  resorcin-blau  belong  to  the 
same  group  as  those  previously  examined,  being  organic  dye-stuffs  which 
show  fluorescence  in  solution.  The  fluorescence  of  these  solutions  was  of 
sufficient  intensity  to  permit  of  the  use  of  the  method  already  described. 
Two  nearly  monochromatic  regions  of  the  spectrum  were  selected  in  each 
case,  one  lying  well  within  the  absorption  band  of  the  solution  and  the  other 
as  far  toward  the  red  as  it  was  possible  to  go  without  reducing  fluorescence 
to  an  extent  which  rendered  measurements  inexact.  The  fluorescence 
spectrum  was  observed  as  before  by  means  of  a  Lummer-Brodhun  spectro- 


100 


B 


»f 


Fig.  1  1  .  —  Rhodamin. 


.       .  . 

Curve  A.  Fluorescence  spectrum  when  excited  by  green  light  in  region  marked  A. 
Curve  B.   Fluorescence  spectrum  when  excited  by  blue  light  in  region  marked  B. 
Curve  T.  Transmission  spectrum.     Layer  i.i  cm.  thick. 

photometer  from  a  direction  at  right  angles  to  that  of  the  exciting  ray.  The 
solution  was  placed  in  a  glass  cell  with  plane  faces,  the  exciting  light  being 
introduced  through  a  slit  and  passing  through  the  layer  of  liquid  lying  next 
to  the  wall  of  the  cell  through  which  the  fluorescent  light  passed. 

Figures  n  and  12  give  in  graphical  form  the  results  of  these  measure- 
ments. The  curves  of  the  fluorescence  spectra  are  of  the  same  type  as  those 
of  fluorescein,  eosin,  and  naphthalin-roth  already  cited.  They  rise  from 
within  the  region  of  the  absorption  band  to  a  well-defined  maximum,  which 
is  located,  generally  speaking,  near  the  infra  edge  of  the  absorption  band; 
beyond  the  maximum  they  fall  away  rapidly  with  increasing  wave-length. 
The  comparison  spectrum  in  these,  as  in  the  previous  cases,  was  that  ob- 
tained from  the  light  diffusely  reflected  by  a  block  of  magnesium  carbonate 
illuminated  by  an  acetylene  flame. 


A   SPECTROPHOTOMETRIC   STUDY   OF   FLUORESCENCE.  13 

Neither  of  these  solutions  conforms  even  approximately  with  the  law  of 
Stokes.  Curve  A  of  Fig.  12  (resorcin-blau)  was  obtained  by  means  of  light 
none  of  which  was  of  shorter  wave-length  than  0.642  //,  whereas  the  fluo- 
rescence is  of  measurable  intensity  to  600  /i.  The  rhodamin  solution  affords 
a  striking  example  of  the  non-validity  of  the  law ;  for  although  the  maximum 
of  the  fluorescent  spectrum  lies  at  0.554  M>  an  observable  fluorescence  was 
produced  by  means  of  very  nearly  monochromatic  light,  obtained  by  the 
method  of  double  dispersion,  using  two  spectrometers  in  series,  the  shortest 
wave-length  of  which  was  0.602  /x.  This  limiting  wave-length  is  indicated 
by  the  line  marked  I  in  Fig.  1 1 .  This  substance,  like  eosin,  which  has  its 
maximum  of  fluorescence  at  0.580  /x,  will  respond  to  the  excitation  of  light 
of  the  D  lines  of  sodium  or  to  even  longer  waves  if  of  sufficient  intensity. 

The  other  substances  to  be  considered  in  this  chapter  are  more  difficult 
subjects  for  quantitative  work  because,  being  less  strongly  fluorescent,  it 


100 


Fig.  12. — Resorcin-blau  in  methyl  alcohol. 

Curve  A.  Fluorescence  spectrum  when  excited  by  red  light  lying  in  region  marked  A. 
Curve  K.  Fluorescence  spectrum  when  excited  by  orange  light  in  region  marked  B. 
Curve  C.  Transmission  spectrum  of  layer  i.i  cm.  thick. 

is  necessary  to  have  recourse  to  powerful  illumination,  and  because  in 
several  instances  the  fluorescence  band  lies  near  the  limits  of  the  visible 
spectrum — either  in  the  violet  or  in  the  extreme  red. 

QUININE   SULPHATE. 

The  quinine  sulphate  subjected  to  measurement  was  prepared  by  making 
a  saturated  solution  in  water  containing  a  trace  of  sulphuric  acid.  The 
solution  was  exposed  to  diffuse  daylight  in  a  cubical  cell  of  glass,  the  faces 
of  which  measured  8  cm.,  and  the  fluorescence  was  observed  through  a  slit 
in  one  of  the  faces  at  right  angles  to  that  exposed  to  the  light.  This  slit  was 
placed  as  near  the  corner  of  the  cell  as  possible,  so  as  to  bring  the  layer  of 
liquid  lying  next  the  glass  on  the  exposed  side  into  the  field  of  view.  The 


14  STUDIES  IN  LUMINESCENCE. 

reference  spectrum,  as  usual,  was  obtained  from  the  light  diffusely  reflected 
from  a  block  of  magnesium  carbonate,  but  in  this  case  the  surface  of  the 
block  was  illuminated  by  daylight,  so  that  the  fluctuations  of  intensity 
might  affect  both  spectra  equally  and  in  the  same  sense. 

The  transmission  of  the  solution  was  determined  by  placing  a  second 
block  of  magnesium  carbonate  behind  the  cell  and  determining  the  intensity 
of  the  light  transmitted  by  the  liquid  in  the  various  regions  as  compared 
with  that  reflected  from  the  first  block.  The  instrument  used  in  this  and 
in  some  of  the  subsequent  measurements  described  in  this  chapter  was  a 
spectroscope  whose  collimator  was  furnished  with  a  double  Vierordt  slit. 
Light  for  the  comparison  spectrum  was  introduced  by  means  of  a  right- 
angled  prism  and  the  adjustment  of  intensities  was  made  in  the  usual 
manner  by  means  of  micrometer  screws. 

A  second  set  of  measurements  with  this  substance  was  made  with  the 
Lummer-Brodhun  spectrophotometer.  A  mercury-arc  lamp  was  used  in 
place  of  the  Nernst  filament  of  our  previous  experiments  and  the  ultra- 
violet line  0.365  ju,  isolated  by  dispersion  with  the  large  spectrometer,  was 
employed  for  excitation.  The  comparison  spectrum  in  this  case  was  the 
magnesium  block  illuminated  by  means  of  the  acetylene  flame. 

It  is  obvious  that  owing  to  the  different  distribution  of  intensities  in  the 
spectra  of  daylight  and  of  the  acetylene  flame  these  two  sets  of  observations 
would  not  be  strictly  comparable.  In  order,  however,  to  make  them  at 
least  approximately  so,  the  following  correction  was  applied  to  the  obser- 
vations by  daylight.  From  certain  data  obtained  by  Vogel1  and  published 
by  him  many  years  ago,  a  curve  was  plotted  giving  the  distribution  of 
intensities  in  the  spectrum  of  diffuse  daylight  with  clouded  sky,  the  spectrun 
of  a  petroleum-gas  flame  being  taken  as  the  standard.  Vogel's  measure- 
ments agree  as  well  as  could  be  expected,  considering  the  fluctuating  char- 
acter of  daylight,  with  those  subsequently  made  by  W.  H.  Pickering2  and 
by  Nichols  and  Franklin.3  As  the  sky  was  overcast  at  the  time  of  making 
the  observations  on  quinine  sulphate  just  described,  it  was  assumed  that 
the  quality  of  daylight  would  on  that  occasion  be  represented  with  sufficient 
accuracy  by  means  of  this  curve.  By  means  of  a  similar  curve,  giving  the 
distribution  of  intensities  in  the  spectrum  of  the  acetylene  flame  as  com- 
pared with  the  petroleum  flame,  it  is  possible  to  determine  the  relation 
between  daylight  and  the  acetylene  flame.  Vogel's  curve  for  daylight- 
petroleum  and  the  computed  curve  for  daylight-acetylene  are  given4  in  Fig.  13. 
By  means  of  the  latter  it  is  possible,  from  the  observations  upon  quinine 
sulphate,  to  compute  a  curve  in  which  the  light  of  the  acetylene  flame 
reflected  from  magnesium  carbonate  is  the  standard.  This  curve  (B,  Fig. 
14)  corresponds  precisely,  as  regards  the  location  of  its  maximum,  with 
curve  A  in  the  same  figure,  which  shows  the  results  of  the  measurement  of 
the  fluorescence  of  this  substance  when  excited  by  means  of  the  ultra-violet 
of  the  mercury  lamp.  Since  these  two  sets  were  made  with  different  types 
of  spectrophotometer  their  agreement  affords  a  desirable  check  upon  the 
performance  of  the  two  instruments. 

lVogel,  Berliner  Monatsberichte,  p.  801,  1880. 

SW.  H.  Pickering,  Proc.  Am.  Acad.  of  Arts  and  Sciences,  vol.  25,  1880. 
'Nichols  and  Franklin,  American  Journal  of  Science,  vol.  38,  p.  100,  1889. 

4The  two  sources  being  so  adjusted  as  to  have  the  same  intensity  at  the  D  line,  the  curve  shows  the  ratio 
of  intensities  for  other  wave-lengths. 


A  SPECTROPHOTOMETRIC   STUDY  OF  FLUORESCENCE. 


Fig.  13. 

Curves  for  daylight-petroleum  and  daylight-acetylene. 


.5/1 


Fig.  14.  —  Quinine  sulphate  in  water. 

Curve  A.  Fluorescence  spectrum  when  excited  by  Hg  line,  0.3650  n. 
Curve  B.  Fluorescence  spectrum  when  excited  by  daylight. 
Curve  T.  Transmission  spectrum.     Layer  8  cm.  thick. 


1 6  STUDIES  IN  LUMINESCENCE. 

The  curve  T  gives  the  transmission  of  the  solution  as  measured  by 
the  method  already  described.  It  was  not  found  possible,  owing  to 
the  fact  that  this  solution  remains  transparent  almost  to  the  limits  of 
the  visible  spectrum,  to  go  very  far  into  the  absorption  band;  but  the 
observations  suffice  to  locate  the  infra  edge  of  the  band  very  closely.  To 
gain  further  information  concerning  the  nature  of  the  absorption  band  a 
series  of  photographs  of  the  transmission  spectrum  were  taken  by  means  of 
a  Rowland  grating  with  sunlight  as  the  source  of  illumination.  The  infra 
edge  of  the  band,  as  shown  by  the  disappearance  of  theFraunhofer  lines,  was 
found  to  lie  in  the  region  indicated  by  the  spectrometric  measurements 
shown  in  Fig.  14.  Absorption  became  almost  complete  in  the  neighborhood 
of  the  H  lines  and  continued  throughout  the  ultra-violet,  at  least  up  to  the 
point  where  glass  becomes  opaque. 

In  order  to  determine  if  possible  the  position  of  the  ultra  edge  of  the  band, 
the  quinine  solution  was  placed  in  a  cell  with  quartz  walls  and  photographs 
were  taken,  using  an  arc  light  into  which  zinc  had  been  introduced  as  a 
source.  Lines  due  to  this  metal  were  distinguishable  in  the  comparison 
spectrum  as  far  as  wave-length  0.2558  n,  to  which  point  the  opacity  of  the 
solution  of  quinine  sulphate  was  found  to  be  complete. 

It  will  be  seen  from  the  curves  A  and  B  (Fig.  14)  that  the  fluorescence 
spectrum  of  quinine  sulphate  is  of  the  same  type  as  that  of  the  various 
fluorescent  dye-stuffs.  It  consists  of  a  single  band  with  a  sharply  defined 
maximum  at  0.437  M-  Since  the  curves  obtained  with  daylight  and  with 
monochromatic  light  from  the  mercury  arc  are  identical  as  regards  the 
position  of  the  maximum  and  in  general  form,  and  since  they  are  of  the 
same  type  as  the  various  other  fluorescence  spectra  already  described,  it  is 
evident  that  quinine  sulphate  belongs  to  the  same  class  as  fluorescein,  eosin, 
etc.  It  is  true  that  in  the  case  of  this  substance  it  is  possible  to  trace  the 
fluorescent  light  throughout  nearly  the  entire  spectrum,  a  point  which  is  to 
be  considered  further  in  a  subsequent  paragraph,  but  beyond  0.5  ^i  intensities 
are  very  small. 

Contrary  to  the  view  expressed  by  Lommel,  the  fluorescence  of  quinine 
sulphate  appears  to  be  independent  of  the  wave-length  of  the  exciting 
light.  Since,  however,  this  is  one  of  the  substances  which  has  been  cited 
in  support  of  Stokes's  law,  it  was  deemed  important  to  determine  as  care- 
fully as  possible  the  longest  wave-length  of  monochromatic  light  capable  of 
exciting  fluorescence.  This  is  a  matter  of  considerable  difficulty  on  account 
of  the  weakness  of  the  effect.  By  means  of  the  method  of  double  dispersion 
already  described,  the  electric  arc  being  used  as  a  source,  monochromatic 
light  thoroughly  free  from  all  stray  radiation  was  obtained.  When  this 
was  used  for  excitation  the  last  trace  of  observable  fluorescence  was  found 
to  disappear  at  wave-length  0.420  /u. 

In  no  other  case  have  we  found  this  limiting  wave-length  to  lie  so  close 
to  the  ultra  edge  of  the  fluorescence  band.  It  will  be  noted,  however,  that 
in  making  measurements  by  daylight  readings  were  obtained  at  wave- 
length 0.41  IJL  and  the  fluorescence  under  strong  excitation  is  traceable  still 
further  into  the  violet,  certainly  almost,  if  not  quite,  to  0.40  /u.  While 
quinine  sulphate  then  approaches  more  nearly  to  conformity  with  Stokes's 
law  than  the  other  substances  that  we  have  studied,  the  evidence  is  dis- 


A   SPECTROPHOTOMETRIC   STUDY   OF   FLUORESCENCE.  17 

tinctly  in  favor  of  the  view  that  it,  like  other  fluorescent  materials,  is 
capable  of  being  excited  by  all  wave-lengths  of  light  lying  in  or  on  the 
infra  edge  of  its  absorption  band ;  and  that  the  longest  wave-length  capable 
of  producing  an  observable  fluorescence  is  appreciably  less  refrangible  than 
the  shorter  wave-lengths  of  its  fluorescence  spectrum. 

CHLOROPHYLL. 

The  fluorescence  spectrum  of  chlorophyll  was  studied  by  Stokes,1  who 
found,  in  addition  to  the  usual  red  band,  a  fainter  excitation  in  the  green 
of  the  spectrum.  Hagenbach2  subsequently  made  an  exhaustive  study  of 
this  substance.  It  was  his  opinion  that  it  did  not  conform  to  the  law  of 
Stokes.  Lommel3  placed  chlorophyll  in  his  first  class,  to  which  belong  all 
substances  whose  fluorescence  is  independent  of  the  wave-length  of  the 
exciting  light. 

The  solution  of  chlorophyll  used  in  our  measurements  was  made  by 
digesting  green  leaves  in  absolute  alcohol  and  filtering.  The  transmission 
curve  (Fig.  15)  shows  four  well-defined  bands,  of  which  the  one  to  which 


.5/i  -6/i  .7fi 

Fig.  15. — Chlorophyll  (fresh  alcoholic  solution  from  green  leaves). 

Curve  A .  Fluorescence  spectrum  when  excited  by  Hg  arc. 
Curve  T.  Transmission  spectrum  of  a  layer  i.i  cm.  thick. 

fluorescence  is  due  is  more  intense  and  broader  than  the  others.  Measure- 
ments made  in  the  extreme  red  appeared  to  indicate  the  presence  of  still 
another  region  of  diminished  transparency  which  it  was  not  possible  to  map 
with  the  spectrophotometer.  In  the  hope  of  determining  more  definitely 
the  character  of  this  band  and  of  ascertaining  whether  the  series  of  striking 
absorption  bands  in  the  visible  spectrum  extends  into  the  infra-red,  we 
requested  Mr.  W.  W.  Coblentz,  who  was  engaged  in  the  study  of  the  infra- 
red by  means  of  a  radiometer  and  a  mirror  spectrometer  with  rock-salt 

Stokes,  Philos.  Trans.,  1853. 

5Hagenbach,  Poggendorfif's  Ann..  141,  p.  245. 

3I,ommeI,  /.  c. 


i8 


STUDIES   IN   LUMINESCENCE. 


prism,  to  explore  the  spectrum  of  this  solution.  The  results  of  his  measure- 
ments are  plotted  in  Fig.  16.  His  observations  extend  from  0.61  ju  to  1.45  ju, 
at  which  wave-length  the  alcohol  in  which  the  chlorophyll  was  dissolved 
becomes  so  nearly  opaque  as  to  prevent  further  readings.  The  absorption 
band  at  0.675  n  is  clearly  shown;  also  the  band  at  the  extreme  edge  of  the 
visible  red  0.745  M-  The  latter,  however,  is  of  little  intensity.  In  Mr. 
Coblentz's  measurements  comparison  was  made  between  a  cell  filled  with 
the  chlorophyll  solution  and  the  same  cell  when  filled  with  alcohol  in  which 
no  chlorophyll  had  been  dissolved.  The  curve  therefore  indicates  the 
effect  of  the  chlorophyll  and  other  dissolved  matters  upon  the  transparency 
of  the  alcohol.  It  will  be  noted  that  between  0.8  n  and  0.9  n  the  material 
in  solution  has  no  absorbing  power.  At  the  latter  wave-length  absorption 
again  begins  to  show  itself  and  increases  steadily  to  1.45  n,  where  the  trans- 
mitting power  of  the  solution  is  only  60  per  cent  as  great  as  that  of  the 
alcohol  itself. 


10051 


.6/1 


Transmission  of  chlorophyll  in  the  infra-red.     (Measurements  by  W.  W.  Coblentz.) 

In  determining  the  fluorescence  curve  A  (Fig.  15),  which  was  obtained 
by  using  the  green  line  of  the  mercury  arc  (0.546  /x)  for  excitation,  we  were 
able  to  trace  the  fluorescence  to  wave-length  0.624  n,  which  lies  well  beyond 
the  ultra  side  of  the  absorption  band.  This  had  not  been  found  possible  in 
the  case  of  the  fluorescence  spectra  previously  described,  partly  on  account 
of  the  greater  width  of  the  bands  and  partly  for  the  following  reason.  The 
fluorescence  spectrum  of  chlorophyll  is  doubtless  traceable  to  an  unusual 
distance  toward  the  violet  because  the  maximum  lies  in  the  extreme  red, 
in  a  region  of  very  low  luminosity.  Fluorescence  in  this  region,  to  be 
appreciable,  must  be  of  great  intensity,  and  on  the  side  toward  the  violet 
the  rapidly  falling  intensity  is  largely  counterbalanced  by  increase  in 
luminosity.  The  same  is  true  of  the  infra  side  of  the  fluorescence  band 
of  substances  like  quinine  sulphate;  so  that,  although  the  curves  are  of 
the  same  type  as  those  described  above — fluorescein,  etc. —  the  fluorescence 


A   SPECTROPHOTOMETRIC   STUDY   OF   FLUORESCENCE.  19 

can  be  followed  nearly  through  the  spectrum.  The  curves  of  resorcin-blau, 
fluorite,  and  aesculin  afford  other  examples  of  this  phenomenon.  Where,  on 
the  other  hand,  the  maximum  is  in  the  middle  of  the  spectrum,  as  in  the  case 
of  rhodamin,  eosin,  fluorescein,  etc.,  the  fluorescence  soon  becomes  too  small 
for  measurement  on  both  sides  of  the  maximum  because  of  the  diminution 
of  luminosity  toward  the  ends  of  the  visible  spectrum. 

The  green  fluorescence  described  by  Stokes  was  faintly  discernible  under 
strong  illumination,  but  its  intensity  would  not  permit  of  measurements. 

The  question  of  the  applicability  of  Stokes's  law  to  this  solution  was 
tested  by  observing  with  monochromatic  light,  by  the  method  already  de- 
scribed, the  longest  wave-length  which  was  capable  of  producing  appreciable 
fluorescence.  The  limiting  wave-length  (0.720  n),  which  gave  faint  but 
unmistakable  fluorescence  (see  line  /,  Fig.  15),  lies  on  the  infra  side  of  the 
maximum  of  the  curve  A. 


.5/J. 


Fig.  17. — Canary  glass. 


Curve  .4.  Fluorescence  spectrum  when  excited  by  violet  light,  X  =0.407  it. 

Curve  B.  Fluorescence  spectrum  when  the  exciting  light  lay  in  the  green,  in  the  region  marked  B. 

Curve  T.  Transmission  spectrum  for  a  layer  7  cm.  thick. 

CANARY  GLASS. 

In  our  experiments  upon  canary  glass  one  corner  of  a  rectangular  slab 
i  cm.  in  thickness  and  7  cm.  wide  was  used.  This  was  mounted  in  front 
of  the  slit  of  the  Lummer-Brodhun  spectrophotometer  in  place  of  and  in  a 
position  corresponding  to  the  cell  employed  in  the  study  of  the  fluorescent 
solutions.  Fluorescence  was  excited  in  the  manner  already  described,  by 
means  of  light  entering  the  glass  at  right  angles  to  the  axis  of  the  collimator 
tube.  Measurements  were  made  using  the  Hg  line  0.407/4  (see  curve  A, 
Fig.  17)  and  green  light  from  the  spectrum  of  the  Nernst  filament  (curve  B). 


20  STUDIES  IN   LUMINESCENCE. 

To  determine  the  location  of  the  absorption  band,  light  transmitted 
through  the  entire  width  of  the  glass,  giving  a  layer  7  cm.  thick,  was  meas- 
ured. The  transmission  curve  T  shows  a  minimum  but  by  no  means 
complete  opacity  at  0.472  ju.  The  absorption  spectrum  of  this  specimen 
did  not  exhibit  the  complex  character  of  that  studied  and  described  by 
Stokes  in  his  memoir  already  cited.  Our  quantitative  exploration,  however, 
did  not  extend  beyond  the  determination  of  the  infra  edge  of  the  band  excit- 
ing fluorescence.  The  fluorescence  spectrum,  contrary  to  expectation,  ap- 
pears to  belong  to  the  same  simple  type  as  that  of  the  fluorescent  substances 
previously  examined.  The  glass  was  optically  very  imperfect  and  the 
fluorescence  measurements  were  seriously  interfered  with  by  the  presence 
of  stray  light.  This  shows  itself  in  the  case  of  curve  B,  which  is  distorted 
in  the  regions  corresponding  in  wave-length  to  the  exciting  light.  The 
observations  upon  the  ultra  side  of  the  curve  are,  as  will  be  seen  from  the 
figure,  considerably  raised  above  the  probable  normal  trend  of  the  curve, 
which  is  shown  by  means  of  the  dotted  line.  Curves  A  and  B,  however, 
have  their  maxima  at  the  same  wave-length,  and,  apart  from  the  discrepancy 
just  mentioned,  their  type  corresponds  precisely  with  that  of  the  numerous 
other  fluorescent  spectra  examined.1 

In  the  determination  of  the  maximum  wave-length  capable  of  exciting 
fluorescence  it  was  especially  necessary  to  distinguish  between  fluorescent 
light  and  ordinary  diffusion  due  to  the  imperfect  optical  properties  of  the 
glass.  When  one  allows  light  to  pass  through  this  glass  after  being  dispersed 
by  a  prism  the  path  of  the  rays  remains  visible  throughout  the  entire  spec- 
trum instead  of  disappearing  for  the  rays  that  do  not  excite  fluorescence,  as 
is  the  case  in  an  optically  perfect  medium.  By  means  of  a  Nicol  prism  held 
before  the  eye  it  was,  however,  easy  to  distinguish  sharply  between  fluo- 
rescence and  ordinary  diffusion.  The  vertical  line  /  marks  the  limiting  wave- 
length (0.539^1)  at  which  fluorescence  becomes  inappreciable.  It  lies  far  to 
the  infra  side  of  the  maximum,  showing  that  in  this,  as  in  all  previous  cases, 
there  was  not  even  approximate  conformity  to  the  law  of  Stokes. 

It  is  probable  that  the  glass  studied  by  Stokes,  and  by  means  of  which 
he  observed  such  extraordinary  fluorescence  phenomena,  was  altogether 
different  from  our  specimen.  It  contained  perhaps  other  fluorescent 
materials  in  solution  which  were  absent  in  the  glass  now  under  considera- 
tion. The  glass  which  we  have  examined  appears  to  belong  in  the  same 
class  with  all  the  other  fluorescent  substances  upon  which  measurements 
were  made:  its  fluorescence  spectrum  consists  of  a  single  band,  the  location 
of  which  is  independent  of  the  wave-length  of  the  exciting  light,  and  it  does 
not  obey  Stokes's  law. 

FMJORITE. 

From  a  collection  of  small  crystals  of  fluorspar  from  Derbyshire,  England, 
placed  at  our  disposal  by  Professor  Gill,  of  the  Department  of  Mineralogy 
of  Cornell  University,  two  specimens  were  selected  for  study.  One  of 
these  was  green  by  transmitted  light,  the  other  colorless.  Both  showed 
the  well-known  blue-violet  color  due  to  fluorescence.  The  transmission 

'For  an  extended  study  of  the  fluorescence  and  absorption  of  another  specimen  of  uranium  glass  see 
R.  C.  Gibbs,  Physical  Review,  xxvin,  p.  361. 


A  SPECTROPHOTOMETRIC  STUDY  OF  FLUORESCENCE. 


21 


curves  of  the  spectrum  of  these  crystals  are  brought  together  for  com- 
parison in  Fig.  1 8.  Curve  A  of  the  green  spar  (thickness  0.8)  cm.  contains 
a  broad,  well-marked  band  in  the  yellow  and  a  second  rather  narrow  band 
to  which  the  fluorescence  is  due.  The  maximum  of  absorption  in  the  latter 
band  is  at  0.424  /z.  Curve  B,  which  relates  to  the  white  crystal,  contains 
only  the  second  band.  This  crystal,  although  thicker  (1.72  cm.),  is  more 
transparent,  the  absorption  in  the  middle  of  the  band  being  less  than 
50  per  cent.  It  is  interesting  to  note  the  presence  of  easily  measurable 
fluorescence  accompanying  such  feeble  absorption. 

The  fluorescence  of  the  green  crystal  was  determined  by  means  of  the 
undispersed  light  of  the  carbon  arc  (curve  A,  Fig.  19)  and  of  the  mercury 
arc  direct  (curve  B).  The  effective  light  from  the  latter  source  comes 
chiefly  from  the  lines  0.407^1  and  0.365/1.  The  fluorescence  conforms  strictly 


foo 


.5u 


Fig.  1 8. 


Transmission  spectra  of  green  fluorspar  (A)  and  white  fluorspar  (B). 

to  that  of  all  substances  previously  examined  by  us.  The  spectrum  consists 
of  a  single  band,  the  maximum  of  which  at  0.433  M  is  independent  of  the 
character  of  the  exciting  light. 

In  the  measurement  of  the  fluorescence  of  the  white  crystal  diffuse 
daylight  was  employed,  and  the  observations  were  made  with  the  spectro- 
scope with  a  single  collimator  tube  and  Vierordt  slit,  already  described. 
The  two  sets  of  readings  obtained  are  plotted  in  Fig.  20.  The  curves 
correspond  very  closely  in  character  with  those  obtained  with  the  green 
fluorite,  although  the  maximum  appears  to  be  shifted  very  slightly  toward 
the  violet.  This  shift  may  be  due  to  a  combination  of  errors  of  observation 
and  of  instrumental  errors,  the  instruments  employed  being  different 
throughout,  or  it  may  result  from  some  slight  difference  in  the  character 
of  the  absorption  band.  It  will  be  noticed  by  reference  to  Fig.  18  that  the 
infra  edge  of  the  absorption  band  of  the  green  crystal  does  not  rise  so 


22 


STUDIES  IN  LUMINESCENCE;. 


r 


Fig.  19. — White  fluorite. 

Curve  A.  Fluorescence  spectrum  when  excited  by  the  carbon  arc. 
Curve  B.  Fluorescence  spectrum  when  excited  by  Hg  arc. 
Curve  T.  Transmission  spectrum  of  a  layer  0.8  cm.  thick. 


Fig.  20. — Green  fluorite. 

Curve  A.  Fluorescence  spectrum  when  excited  by  diffuse  daylight. 

Curve  B.  The  same  repeated  on  another  day. 

Curve  T.  Transmission  spectrum  of  a  layer  1.72  cm.  thick. 


A   SPECTROPHOTOMETRIC   STUDY   OF   FLUORESCENCE.  23 

sharply  as  is  the  case  of  the  white  fluorite.  The  broadening  of  the  band 
upon  this  side  may  result  in  a  slight  shifting  of  the  fluorescence.  It  has 
already  been  shown  by  Stenger1  and  by  O.  Knoblauch2  that  fluorescent 
substances  such  as  eosin  and  naphthalin-roth  have  the  position  of  the  fluo- 
rescent spectrum  shifted  by  dissolving  them  in  different  liquids ;  and  that 
the  shift  corresponds  with  that  of  the  absorption  band  previously  described 
by  Kundt.3 

^SCULIN. 

This  substance  was  cited  by  Lommel4  as  typical  of  his  second  class, 
the  characteristics  of  which  are  a  fluorescence  spectrum  independent  of 
the  wave-length  of  the  exciting  source  and  an  absorption  band  beginning 
at  that  point  in  the  spectrum  at  which  the  first  trace  of  fluorescence  shows 


Fig.  21. — Jjsculin  (from  horse  chestnut). 

Curve  .4.  Fluorescence  spectrum  when  excited  by  arc  light. 
Curve  T.  Transmission  spectrum  of  a  layer  8  cm.  thick. 

itself  and  extending  toward  the  violet,  so  that  there  is  no  overlapping  of 
the  band  of  the  exciting  light  with  the  fluorescence  spectrum. 

Our  measurements  of  the  fluorescence  of  aesculin  were  made  upon  a 
solution  consisting  of  water  in  which  freshly  broken  twigs  of  the  horse- 
chestnut  tree  had  been  immersed.  When  freshly  prepared  this  shows  the 
well-known  fluorescence  characteristic  of  aesculin,  but  the  solution  loses 
its  activity  upon  standing.  We  were  unable  to  procure  pure  aesculin,  but 
the  results,  so  far  as  the  form  and  composition  of  the  fluorescence  curve 
are  concerned,  would  probably  be  the  same  had  measurements  been  made 
upon  the  solution  of  the  chemically  prepared  substance.  The  fluorescence 
curve,  Fig.  21,  is  of  the  usual  form  and  its  position  with  reference  to  the 


'Stenger,  Wied.  Ann.,  28,  p.  201. 

2O.  Knoblauch,  Wied.  Ann.,  54,  p.  193. 


'Kundt,  Wied.  Ann..  4.  p.  34,  1878. 
•Lommel,  Poggendorff's  Ann.,  143,  p.  38. 


24  STUDIES   IN   LUMINESCENCE. 

transmission  curve  T  is  similar  to  that  of  the  substances  previously  examined. 
On  the  infra  side,  as  has  been  pointed  out  by  Lommel,  it  is  possible  to 
trace  the  fluorescence  throughout  the  entire  spectrum ;  but  this,  as  in  the 
instances  already  considered,  is  due  to  the  greater  luminosity  as  we  move 
toward  the  middle  of  the  spectrum  rather  than  to  any  unusual  maintenance 
of  the  intensity  of  the  fluorescence  in  that  direction.  The  band  falls  off 
quite  sharply  on  that  side,  differing  not  at  all  in  this  respect  from  the  bands 
of  other  substances,  such  as  quinine  sulphate  and  fluorite.  The  measurement 
of  the  transmission  showed  a  very  dilute  solution,  in  which  the  maximum 
of  absorption  is  scarcely  more  marked  than  in  the  case  of  the  bands  of  the 
specimens  of  fluorite  already  described. 

From  the  location  of  the  absorption  band  with  reference  to  the  maximum 
of  fluorescence  it  would  be  safe  to  predict  for  this  substance,  on  the  basis 
of  previous  experience,  a  wide  divergence  from  the  law  of  Stokes.  Deter- 
mination of  the  limiting  value  of  the  exciting  light  is  somewhat  difficult 
to  make.  It  was  found,  using  the  spectrum  of  the  arc  lamp  as  a  source, 
that  the  fluorescence  due  to  light  of  wave-length  0.45  /*  could  still  be 
discerned. 

It  is  not  possible,  with  the  spectrophotometer,  to  make  a  complete 
exploration  of  the  absorption  band  of  aesculin,  since  the  band  extends  into 
the  ultra-violet.  Photographs  of  the  absorption  spectrum  of  this  solution, 
made  by  Miss  Eleanor  Burns  at  our  suggestion,  show  this  band  to  be  com- 
paratively narrow,  its  ultra  edge  lying  at  about  0.34  n,  beyond  which  there 
is  no  absorption  capable  of  detection  by  photographic  means. 

There  is  nothing  in  these  measurements  which  would  lead  one  to  place 
aesculin  in  a  class  different  from  that  of  the  foregoing  substances.  It  is 
distinguished,  as  regards  the  character  of  its  fluorescence  spectrum,  from 
quinine  sulphate  and  fluorite,  the  other  two  substances  on  our  list  the  fluo- 
rescence of  which  lies  in  the  blue,  only  in  that  the  maximum  lies  at  a  some- 
what greater  wave-length.  The  curve  A,  Fig.  21,  was  obtained  by  the  use 
of  the  direct  light  of  the  arc,  undispersed.  Since  the  curve  corresponded 
in  every  form  to  the  general  type  it  did  not  seem  necessary  to  repeat  the 
observations  with  exciting  light  of  other  composition. 

SUMMARY. 

From  the  measurements  described  in  this  chapter  the  following  conclu- 
sions may  be  drawn: 

1.  Eosin,  naphthalin-roth,  fluorescein,  rhodamin,  resorcin-blau,  quinine 
sulphate,  chlorophyll,  canary  glass,  green  fluorspar,  white  fluorspar,  and 
aesculin  all  exhibit  fluorescence  of  the  same  type. 

2.  The  fluorescence  spectrum  in  general  consists  of  a  single  band  situated 
near  the  infra  edge  of  the  absorption  band  with  which  fluorescence  is 
associated. 

3.  The  position  of  the  maximum  of  the  fluorescence  band  is  in  all  cases 
independent  of  the  wave-length  or  composition  of  the  exciting  light. 

4.  The  distribution  of  intensities  in  the  fluorescence  spectrum  is  inde- 
pendent of  the  wave-length  of  the  exciting  light. 

5.  Stokes'slaw  holds  for  none  of  the  numerous  fluorescent  substances 
thus  far  examined. 


CHAPTER  II. 

ON  THE  ABSORBING  POWER  AND  THE  FLUORESCENCE  OF 

RESORUFIN. 

The  experiments  described  in  this  chapter  were  undertaken  at  the 
suggestion  of  the  authors  by  Miss  Frances  G.  Wick.  An  account  of  them 
was  first  given  in  the  Physical  Review.1  The  chief  object  was  to  determine 
whether  the  typical  fluorescence  spectrum  changes  with  the  concentration 
of  the  solution  or  whether  the  shift  of  the  band  referred  to  in  Chapter  I  is 
caused  by  absorption  only.  The  substance  studied  was  resorufin,  whose 
strong  fluorescence  in  the  yellow-red  is  conveniently  excited  and  readily 
observed. 

It  has  frequently  been  observed  that  the  color  of  the  light  emitted  by  a 
fluorescent  solution  is  altered  by  a  change  in  the  concentration  of  the 
solution.  A  dilute  solution  of  fluorescein,  as  indicated  in  Chapter  I,  gives 
a  green  fluorescence,  while  the  light  emitted  by  a  concentrated  solution  of 
the  same  substance  shows  a  distinctly  yellow  tinge.  In  other  words,  an 
increase  in  concentration  causes  the  maximum  of  the  fluorescence  spectrum 
to  shift  toward  the  longer  wave-lengths. 

Such  a  shift  might  result  from  a  real  change  in  the  period  of  vibration  of 
the  fluorescent  molecules,  i.  e.,  the  form  of  the  typical  fluorescence  spectrum 
might  depend  upon  the  concentration  of  the  solution.  But  the  observed 
change  in  the  fluorescence  spectrum  might  equally  well  result  from  a  change 
in  the  absorbing  power  of  the  solution.  The  light  emitted  by  portions  of 
the  active  substance  in  the  interior  of  the  solution  must  pass  through  the 
solution  before  emerging,  and  is  therefore  weakened  by  absorption. 
Since  the  maximum  of  the  fluorescence  spectrum  does  not  occur  at  the 
same  wave-length  as  the  maximum  absorption,  being  always  slightly  dis- 
placed in  the  direction  of  the  longer  waves,  it  is  clear  that  different  portions 
of  the  fluorescence  spectrum  will  be  affected  by  absorption  in  varying 
degree;  the  shorter  waves  will  always  be  absorbed  most  strongly.  The 
maximum  of  the  fluorescence  spectrum  is  therefore  always  shifted  toward 
the  red  to  some  extent,  and  the  increased  absorbing  power  of  concentrated 
solutions  causes  the  shift  due  to  this  cause  to  increase  with  the  concentration. 

The  experimental  work  naturally  falls  under  three  heads,  as  follows: 

1.  The  relation  between  absorption  and  thickness,  to  test  whether  or 
not  the  absorption  of  a  fluorescent  solution  obeys  the  exponential  law  of 
optically  perfect  substances. 

2.  The  relation  between   absorption   and  concentration,   to  test  the 
application  of  Beer's2  law,  /'.  e.,  that  an  increase  in  the  concentration  of  a 
solution  is  equivalent  to  an  increase  in  thickness. 

3.  The  study  of  fluorescence  spectra  of  solutions  of  resorufin  of  different 
concentrations  to  determine  whether  or  not  there  is  any  shift  in  the  maxi- 
mum of  fluorescence  as  concentration  changes. 


'Frances  G.  Wick,  Physical  Review,  xxiv,  p.  356. 
:Beer,  Poggendorff's  Ann.,  86,  p.  78. 

25 


26  STUDIES   IN   LUMINESCENCE. 

HISTORICAL. 

In  1888  B.Walter,1  using  the  Vierordt  spectrophotometer,  found  the  ratio 
of  the  fluorescent  light  emitted  to  total  incident  light  absorbed,  Fl/A,  to 
increase  with  dilution.  This  law  had  been  previously  conjectured  by 
IvOmmel,2  whose  mathematical  theory  Walter3  used  in  verifying  his  own 
results.  In  connection  with  this  work  Walter  tested  Beer's  law  of  absorp- 
tion for  fluorescein  by  four  measurements.  He  found  the  law  to  hold  true 
for  dilute  but  not  for  concentrated  solutions. 

Walter  briefly  states  his  conclusions  as  follows : 

"Ability  to  excite  fluorescence,  Fl/A,  in  the  most  concentrated  solutions, 
is  infinitely  small  or  zero.  After  Fl/A  obtains  a  measurable  value  it 
increases  in  proportion  to  dilution  to  a  certain  dilution  called  the  'critical 
point.'  For  greater  dilutions  Fl/A  remains  constant." 

In  explanation  of  the  above,  Walter4  advances  a  molecular  group  theory 
which  is  of  interest,  preceding  as  it  does  the  present  theory  of  ionization 
given  by  Buckingham.5  Walter  summarizes  his  theory  in  the  following 
statements : 

1.  Every  separate  molecule  of  a  given  substance  in  solution  absorbs, 
so  long  as  it  remains  in  the  separate  condition,  the  same  fraction  of  the  light 
falling  upon  it,  no  matter  how  great  its  distance  from  other  molecules  in  the 
solution  may  be. 

2.  Every  separate  molecule  of  a  given  fluorescent  substance  in  solution, 
so  long  as  it  is  in  the  separate  condition,  changes  the  same  fraction  of 
absorbed  light  into  fluorescent  light,  no  matter  how  great  its  distance  from 
neighboring  molecules  may  be. 

3.  As  soon  as  molecules  begin  to  combine  in  groups  the  validity  of  these 
statements  ceases.     Fluorescence  entirely  ceases  in  such  a  group  and 
absorption  extends  over  wave-lengths  which  a  separate  molecule  is  not  able 
to  absorb. 

Walter's  idea  of  the  separate  molecule  seems  to  correspond  closely  with 
what  is  now  known  as  the  ion. 

In  1894  H.  Buckingham6  performed  a  series  of  careful  experiments  to 
discover  whether  or  not  fluorescent  substances  are  in  a  state  of  ionization. 
His  investigations  led  to  the  conclusion  that,  at  least  in  some  cases,  fluo- 
rescence is  due  to  ionization  and  produced  only  by  that  part  of  a  solution 
which  is  ionized.  Certain  ions,  along  with  their  other  well-known  optical 
properties,  possess  this  property  of  fluorescence. 

The  conclusions  drawn  from  the  spectrophotometric  work  described  in 
Chapter  I  are  found  to  apply  also  to  resorufin.  For  example: 

1.  The  characteristic  fluorescence  band  is  situated  near  the  edge  of  the 
absorption  band  and  is  steeper  on  the  side  toward  the  violet. 

2.  As  fluorescent  light  passes  through  greater  thicknesses  of  liquid,  the 
position  of  the  maximum  in  the  fluorescence  spectrum  is  shifted  toward 
the  red. 


'B.  Walter.  Wied.  Ann.,  34,  1888.  «E.    Buckingham,    Zeitschrift   fur    physikaliscbe 
2L,ommel,  Poggendorff's  Ann.,  160,  p.  76,  1877.  Chemie,  14,  p.  129,  1894. 

»B.  Walter,  Wied.  Ann.,  36,  p.  502,  1889.  «E.  Buckingham,  I.  c. 
4B.  Walter,  Wied.  Ann.,  36,  p.  518,  1889. 


ABSORBING  POWER  AND  FLUORESCENCE  OF  RESORUFIN. 


3.  The  effect  of  diluting  a  fluorescent  solution  is  similar  to  that  of 
diminishing  the  distance  through  which  the  fluorescent  light  travels. 

METHOD   OF   OBSERVATION. 

The  instrument  used  for  this  work  was  the  spectrophotometer  of  Lummer 
and  Brodhun. 

The  source  of  light  was  an  acetylene  flame  S  (Fig.  22)  from  which  light 
was  diffusely  reflected  by  a  block  of  magnesium  carbonate,  n,  mounted  at  an 
angle  at  45°  with  axis  of  the  collimator  A .  For  observations  on  transmission 
a  similar  block  of  magnesium  carbonate  was  placed  in  front  of  the  slit  D. 

The  work  was  performed  in  a  room  with  black  walls,  the  acetylene  flame 
being  the  only  source  of  light.  Screens  were  adjusted  so  that  no  stray  light 
could  strike  any  part  of  the  instrument. 

The  solution  under  consideration  was  placed  in  front  of  slit  D,  the  width 
of  which  was  kept  constant  during  a 
single  set  of  observations.  The  ocular 
lenses  of  the  telescope  were  removed, 
the  eye  being  focused  upon  the  face 
of  the  prism  magnified  by  the  objec- 
tive of  the  telescope  T. 

In  making  observations  the  width 
of  slit  C  was  adjusted  by  means  of  a 
micrometer  screw  until  the  central 
band  of  light  reflected  from  KL  was 
brought  into  the  best  possible  coinci- 
dence with  that  transmitted  from  D 
through  the  upper  and  lower  parts  of 
the  cube.  The  results  given  represent 
an  average  of  from  5  to  10  settings, 
the  zero  point  of  the  screw  attached  to 
C  being  carefully  determined  and  care 
being  taken  to  avoid  errors  due  to  lost 
motion. 

The  hydrogen  lines  of  a  vacuum 
tube,  together  with  a  few  of  the  Fraun- 
hofer  lines,  were  used  for  calibration. 


Fig.  22. — Apparatus  arranged  for 
measuring  fluorescence. 


COMPOSITION   OF  RESORUFIN. 

Weselski's  diazo-resorufin,1  the  fluorescent  substance  used  in  this  investi- 
gation,2 has  the  formula 


! 


H 


J— OH 


'Berichte  der  deutschen  chemischen  Gesellschaft,  22,  3036. 

2  Acknowledgment  should  be  made  to  Prof.  W.  R.  Orndorff  for  his  kindness  in  preparing  the  resorufin  used 
in  this  work. 


28 


STUDIES  IN   LUMINESCENCE. 


Dissolved  in  absolute  alcohol  it  is  bright  red  in  color,  with  a  sharply  defined 
fluorescence  band  extending  from  0.54  M  to  the  limits  of  the  visible  spectrum 
in  the  red.  The  transmission  spectrum  given  in  Table  2  and  Fig.  23  shows 
the  absorption  band  corresponding  to  this,  and  also  a  smaller  absorption 
band  in  the  violet. 

Different  concentrations  of  resorufin  were  used.  These  are  merely  rela- 
tive and  are  indicated  as  "concentration  £,  |,  f,  ^\,  -£%,  Jf,  and  Y\S>"  the 
original  solution  being  taken  as  \ .  All  curves  corresponding  to  the  above 
concentrations  are  indicated  by  the  letters  A,  B,  C,  D,  E,  F,  and  G  respectively. 

TABLE  2. 

Transmission  through  layer  of  resorufin  1.075  cm.  thick.  Concentration  |.  X  =  Wave- 
length. 70  =  Intensity  of  light  before  transmission.  7= Intensity  of  light  after 
transmission. 


X 

/o 

j 

—  =  Transmission. 

X 

/o 

, 

—  =  Transmission. 

/„ 

/o 

0.384^ 

45 

6 

O.  I  I  1 

0.48/1 

45-72 

32.05 

0.70 

•39 

46 

12 

.261 

.492 

45-72 

28.87 

.63 

•394 

46 

20.25 

.438 

.508 

44-7 

24.4 

.538 

•407 

46 

16 

•347 

.524 

44-7 

'7-57 

•394 

•4«3 

46 

13.62 

.297 

•542 

46.27 

1  1 

.241 

.42 

46 

9-75 

.213 

.562 

46.41 

7-52 

.162 

•429 

46 

9-95 

.217 

.589 

46.25 

1  1  .64 

•253 

.438 

45-5 

12.1 

.266 

.614 

46.55 

32.27 

.706 

.448 

45-5 

14.15 

•32 

.646 

46.62 

45.62 

.981 

•457 

45-05 

22.27 

-497 

.69 

44.52 

44 

•99 

.468 

45-05 

25-97 

•579 

•72 

41.25 

41 

•  99 

ABSORPTION  AND  THICKNESS. 


The  first  problem  under  consideration  was,  as  stated  above,  to  find 
whether  a  fluorescent  solution  acts  like  an  optically  perfect  substance  in 
obeying  Lambert's  law  of  absorption,  i.  e.,  whether 


where  70  and  I  are  the  intensities  before  and  after  transmission,  and  x  is 
the  thickness  of  the  absorbing  layer. 


( 

!i 


tr-% 


\ 


W.v.   l_.r,gtK    ' 

Fig.  23. 
Transmission  spectrum  of  layer  1.075  cm.  thick.     Concentration  J. 


ABSORBING  POWER  AND  FLUORESCENCE  OF  RESORUFIN. 


To  test  this  law  the  coefficient  of  absorption,  /3,  was  calculated  from 
observations  taken  with  cells  of  different  inside  thickness,  the  measurements 
of  these  cells  being  made  with  great  care. 

The  larger  cell  was  measured  with  micrometer  calipers.  Thickness  of 
glass  =  a,  |8  (Fig.  24).  Outside  thickness  of  that  part  of  the  cell  used  for 
transmission  =  C.  Inside  thickness  =  T. 

r  =  C-(a+/3)  =  2.05  cm. 

In  the  case  of  small  cell  (Fig.  25),  calipers  could  not  be  used  to  measure 
the  thickness  of  glass.  The  cell  was  placed  under  a  traveling  microscope 
to  measure  the  average  inside  thickness  of  the  top.  The  outside  thickness 
at  the  points  indicated  in  the  figure  was  measured  with  calipers. 

T=T'-\-(n—m)  =  1.075    cm- 

It  was  assumed  that  the  thickness  of  glass  from  top  to  bottom  of  cell  re- 
mained constant,  since  broken  pieces  of  the  same  glass  varied  not  more  than 
o.oi  mm. 

To  determine  the  transmission,  the  cell  containing  the  resorufin  was 
placed  in  front  of  slit  D  (Fig.  22),  care  being  taken  that  no  direct  light 
from  S  should  strike  the  cell,  and  light  from  the  block  of  magnesium  car- 


a 


<-  ----  T  ----  -> 


<--- 

(.—  T—  •» 
—  m  

— 

(.  —  T  > 

t-- 

n  

—  * 

Fig.  24.  Fig.  25. 

bonate,  in  front  of  slit  D  (not  shown  in  the  figure),  was  transmitted  through 
it.  The  reading  of  C  was  taken  to  get  the  relative  intensity,  7,  of  the  light 
transmitted  for  different  wave-lengths.  After  each  observation  the  cell 
containing  resorufin  was  removed  and  an  identical  one  filled  with  absolute 
alcohol  put  in  its  place.  The  reading  of  C  in  this  case  gave  the  intensity  of 
the  incident  light  before  absorption,  70.  From  these  observed  values  of  7o 
and  7,  together  with  the  thickness  of  the  absorbing  layer,  the  coefficient 
of  absorption  was  computed.  If  the  loss  by  reflection  may  be  regarded  as 
the  same  for  the  cell  containing  resorufin  as  for  that  filled  with  alcohol — and 
the  difference  must  be  extremely  small — it  will  be  seen  that  the  influence  of 
reflection  is  to  introduce  a  factor,  i  —  R,  in  the  case  of  both  settings,  and  that 
this  factor  will  disappear  when  the  ratio  is  taken. 

As  a  check  upon  these  results  a  different  method  of  observation  was 
used.  Keeping  D  (Fig.  22)  constant,  the  reading  Ci  was  taken  for  the 
light  alone.  A  reading  of  C2  was  then  taken  with  alcohol  in  front  of  C  and 
resorufin  in  front  of  D. 

Cl 


Ci 
D 


=  Incident  light  (70) 


~  =  Transmitted  light  (7)         —  =  ^  =  L 
•L)  Cz        Cz         7o 


D 


STUDIES  IN  LUMINESCENCE. 


As  a  further  check  the  positions  of  alcohol  and  resorufin  were  reversed,  the 

alcohol  being  placed  in  front  of  D  and  the  resorufin  in  front  of  C.    Then 

D  _  D  _  I    _  Cz 

C-i  C2  -f  0  LI 

It  will  be  noticed  that  in  this  method  also  the  disturbances  due  to  reflec- 
tion from  the  surfaces  of  the  cells  are  eliminated. 

From  these  data  the  value  of  the  coefficient  of  absorption  for  each  wave- 
length was  found  to  be  practically  the  same  as  that  found  by  the  first 


0.4// 


0.5//  0.6/* 

x=  points  for  cell  1.075  cm.  tnick 

•  ••  ••        "        Z.05    "  " 


Fig.  26. 

Coefficient  of  absorption  as  computed  from  cells  of  different  thickness. 

method  and  independent  of  the  thickness  of  the  cell.  Absorption  curves 
plotted  from  average  values  obtained  by  both  methods,  using  cells  of  two 
thicknesses,  are  identical,  as  shown  in  Fig.  26. l  The  absorption  coefficients 
for  different  concentrations  and  also  the  data  from  which  the  computation 
was  made  are  given  in  Table  3. 

TABLE  3. 

Coefficient  of  absorption  computed  from  cells  of  different  thickness.  Concentration 
=  1/16.  70  =  Intensity  of  light  before  transmission.  7=Intensity  of  light  after 
transmission.  0  =  Coefficient  of  absorption. 


X 

Thickness  of  cell  = 

1.075  cm. 

Thickness  of  cell  =  2.05  cm. 

la 

/ 

ft 

/o 

, 

0.429/4 

79-9 

35-8 

0.7476 

8l.8 

\ 
19.9  j  0.6884 

.438 

78.8 

48.4 

•4539 

80.0 

25-9       -5495 

.448 

80.2 

53-6 

•3746 

82.4 

33-7  i     -4363 

•457 

79.5       62.5 

.2223 

80.6 

48.3       .2495 

.468 

81.6  \    66.1 

.1805' 

80.4 

54-5       ->7022 

.48 

78.9      70.0 

.1175- 

82.3 

61.3       .  I2672 

.492 

76.1       64.8 

•15 

80.5 

57.0       .1678 

.508 

76.5       57-5 

.2653 

79-7 

43-'  ;     -2993 

•524 

76.8       45.8 

.4808 

78.9 

29.5       .4786 

.542 

75-6       34-5 

.7283 

78.5 

17.3       .73% 

.562 

75-9 

25.4 

1  .022 

78.7 

10.  o  j  i.  0304= 

.589 

75-8 

30.9 

0.8354 

79-4 

18.9     0.6979 

.614 

75-i 

63.4 

•<579 

79-8 

54.0       .1899 

.646 

73.6 

72.2 

.0169 

80.9 

75.8  i     .0316 

.69 

64.9 

63.8 

.0148 

7'-3 

69.2  j     .0144 

1Figures  26,  27,  and  28  give  only  relative  values  of  coefficients,  since  they  are  drawn  from  data  computed 
upon  the  basis  of  ordinary  instead  of  natural  logarithms.  To  get  from  the  curves  the  absolute  value  of 
the  coefficients  given  in  the  tables  multiply  by  2.3. 

2These  values  are  average  results  from  a  number  of  observations. 


ABSORBING  POWER  AND  FLUORESCENCE  OF  RESORUFIN.       31 

It  appears,  therefore,  that  the  absorption  of  resorufin  is  in  accordance 
with  Lambert's  law.  Deviations  from  this  law,  if  present,  are  at  least  too 
small  to  be  detected  by  the  experimental  methods  used. 


Concentration 
Fig.  27. 

Curves  showing  effect  of  change  in  concentration  upon  coefficient  of  absorption1  of 
wave-lengths  marked  on  curves. 


Me 


Ke 


V* 

Concentration 


Vz 


Fig.  28. 

Average  coefficients  of  absorption1  for  different  concentrations  taken  from  Table  4. 


!S«e  footnote  on  page  30. 


32  STUDIES  IN   LUMINESCENCE. 

ABSORPTION  AND  CONCENTRATION. 

The  second  set  of  experiments  was  undertaken  to  study  the  relation 
between  absorption  and  concentration  and  thus  test  the  validity  of  Beer's 
law  for  a  fluorescent  substance,  i.e.,  "increasing  the  concentration  of  a 
solution  is  equivalent  to  a  like  increase  in  thickness." 

The  same  method  of  observation  was  employed  for  this  as  for  the  pre- 
ceding work,  no  changes  in  the  apparatus  being  necessary.  The  coefficient 
of  absorption  was  found  for  different  concentrations,  the  solution  being 
placed  in  a  cell  2.05  cm.  thick  if  dilute,  and  in  cell  1.075  cm-  thick  if  con- 
centrated. This  change  of  cell  is  permissible,  since,  according  to  the  results 
obtained  above,  L/ambert's  law  of  absorption  applies  to  resorufin.  The 
absorption  coefficient  for  different  wave-lengths  was  thus  obtained  for 
seven  concentrations. 

TABLE  4. 
Coefficient  of  absorption  for  corresponding  wave-lengths  in  different  concentrations. 


X 

I 

1 

i 

A 

3¥         *V 

"1  -.;  t" 

o.4'3M 

2.254 

1.8814 

1.2132 

0.7125 

0.3107 

0.1444 

0.0276 

.42 

2.806 

2.2793 

'•4237 

•8533 

.4360 

.1984 

.0775 

.448 

2-53 

i  .  5364 

0-7935 

•  3565 

.  I  86O     .  0897 

.0368 

.48 

1.336 

0.6934 

0.3864 

•'734 

.0749 

.0586 

.0367 

.524 

3.  162 

1.8584 

1.1247 

•5349 

.2783 

•'453 

.0692 

.562 

3-473 

3.0797 

2.0171 

.989 

•544' 

.3036 

.1442 

.614 

0.3864 

0.3082 

0.1550 

.0995 

.0484 

.0524 

.0265 

Average 

2.2782 

i  .  6624 

i  .0162  + 

0.5313 

0.2683 

0.1417 

0.0597 

Figs.  27  and  28  give  the  results  of  these  observations.  It  will  be  noticed 
from  these  curves  that  the  coefficient  of  absorption,  in  the  case  of  dilute 
solutions,  increases  in  direct  proportion  to  concentration.  For  concentrated 
solutions  this  proportion  fails,  the  concentration  increasing  more  rapidly 
than  the  absorption.  This  is  true  for  all  the  wave-lengths;  in  every  case 
the  curve  starts  out  as  a  straight  line  for  dilute  solutions  and  bends  down- 
ward as  higher  concentrations  are  reached. 


TABLE  5. 

Coefficients  of  absorption  computed  from  average  of  careful  observations  for 
concentration  ^  and  average  absorption  concentration  curve.     (Fig.  28). 


i 

1 

1 

A 

3'f 

A 

128 

0.524/11 

2.1986 

1.60471 

0.9841  !  0.51313 

0.2599 

0.12995 

0.0651 

.542 

3.266 

2.385 

'•455 

0.76245   .38398 

.19251 

.09637 

.562 

4.3608 

3-175 

1.946    1.0177    -5'54 

.25783 

.1288 

.589 

2.7324 

1-995 

1.219    0.63733   .3227 

.  16146 

.O8O7 

.614  i  0.49335 

0.36087 

0.22157  0.115322   .05842 

.0292 

.0146 

.646 

0.95588 

0.0698 

0.0428   0.0223    .0113 

.00575 

.  OO283 

ABSORBING   POWER   AND   FLUORESCENCE;   OF   RESORUFIN.  33 

Fig.  28  shows  the  average  coefficient  of  absorption  for  the  seven  wave- 
lengths as  a  function  of  the  concentration.  This  curve  is  used  later  for 
finding  the  coefficient  of  absorption  for  different  dilutions,  especially  care- 
ful measurements  for  the  concentration  ^  being  taken  as  a  basis  of  cal- 
culation. The  values  thus  obtained,  given  in  Table  5,  were  used  in  all 
subsequent  computations  involving  the  coefficient  of  absorption. 

The  above  results  agree,  in  every  respect,  with  those  given  by  Walter.1 
Beer's  law  is  true  for  dilute  solutions,  but  fails  for  greater  concentrations, 
as  is  indicated  by  the  deviation  of  the  curve  from  a  straight  line.  The 
straight  form  of  the  curve  (Fig.  28)  up  to  concentration  f  corresponds  to 
what  Walter  calls  "complete  solution,"  in  which  he  says  the  molecules  are 
in  a  "separate  state."  Concentration  |  may  be  regarded  as  his  "critical 
dilution,"  where  a  change  seems  to  take  place  in  the  condition  of  the  fluo- 
rescent substance.  Solutions  more  concentrated  than  £  correspond  to  those 
called  by  Walter  "incomplete,"  in  which  "molecular  groups"  exist. 

Interpreted  according  to  the  ionization  theory,  the  curve  (Fig.  28) 
indicates  that  in  dilutions  less  than  £  a  state  of  complete  or  nearly  com- 
plete ionization  has  been  reached.  At  this  point  a  change  takes  place, 
more  resorufin  being  contained  in  the  solution  than  is  ionized.  As  the 
concentration  increases,  more  and  more  of  the  solution  remains  undis- 
sociated.  It  appears  that  the  undissociated  resorufin  is  not  only  incapable 
of  fluorescence,  but  is  also  much  less  effective  in  causing  absorption  than 
is  the  dissociated  substance. 

FLUORESCENCE   AND   CONCENTRATION. 

For  observation  of  the  fluorescence  spectrum  the  spectrophotometer 
was  adjusted  as  before.  In  front  of  slit  D  (Fig.  22)  was  placed  a  glass  cell 
5.4  cm.  long,  containing  the  resorufin  solution.  This  cell  was  entirely 
covered  with  black  paper,  except  for  a  space  about  1.5  cm.  high  across  the 
bottom  of  the  face  next  to  the  exciting  light  S',  and  two  narrow  strips,  x  and  y, 
The  opening  x  was  to  allow  light  to  enter  the  collimator  slit,  while  y, 
directly  opposite,  was  used  only  for  adjusting.  The  source  of  illumination 
used  to  produce  fluorescence  was  a  bank  of  four  acetylene  flames  S'. 
Between  these  and  the  fluorescent  solution  was  placed  a  glass  cell  filled 
with  water  to  prevent  the  heating  of  the  resorufin.  The  comparison  source 
was  another  acetylene  burner  6',  light  from  which  was  reflected  into  the 
slit  C  by  a  block  of  magnesium  carbonate  n.  The  cell  containing  the 
resorufin  was  so  placed  that  light  from  the  whole  layer  of  solution  next  the 
inside  surface  of  glass  came  through  the  slit  D  of  the  collimator. 

It  is  clear  that  a  portion  of  the  fluorescent  light  is  absorbed  by  the  solu- 
tion before  reaching  D,  and  attention  has  already  been  called  to  the  fact 
that  this  absorption  will  be  different  for  different  parts  of  the  fluorescence 
spectrum.  If  the  fluorescence  is  measured  in  the  manner  indicated  above 
it  is  therefore  necessary  to  apply  a  correction  for  absorption  before  the 
typical  fluorescence  spectrum  can  be  determined.  Two  methods  of  making 
this  correction  have  been  used.  In  the  first  method  the  necessary  cor- 
rection was  computed  as  follows : 

>B.  Walter,  Wied.  Ann.,  36,  pp.  502  and  518,  1889. 


34  STUDIES   IN   LUMINESCENCE. 

Let  P  be  any  point  in  the  slit  (Fig.  29).  The  source  of  the  light  reaching 
P  is  a  cone  in  the  fluorescent  liquid  converging  toward  P.  The  angle  of 
this  cone  is  determined  by  the  aperture  of  the  object  glass,  EF,  of  the 
collimator.  Any  part  of  this  cone  cut  off  by  the  glass  walls  of  the  cell  or 
by  the  upper  surface  of  the  solution  is  supplied  by  total  reflection. 

Consider  a  disk  bounded  by  two  circular  sections  of  this  cone  at  distances 
x  and  x+dx  respectively  from  the  apex  P  (Fig.  30).  The  amount  of 
fluorescent  light  reaching  P  from  such  a  disk  as  this  will  be  independent 
of  x,  provided  no  absorption  takes  place,  for,  while  the  area  of  the  section 
varies  as  x2,  the  intensity  of  the  light  reaching  P  from  each  small  portion 
of  the  disk  varies  as  i/x~.  The  fluorescent  light  that  would  reach  P  from 
such  a  disk  if  there  were  no  absorption  is  therefore  kdx,  where  k  is  a  func- 
tion of  the  wave-length,  X. 


Fig.  29.  Fig.  30. 

Since  the  light  emitted  by  each  section  of  the  cone  is  in  part  absorbed, 
we  have  for  the  light  reaching  P  from  one  of  the  disks 


and  the  total  light  reaching  P  from  the  whole  cone  is 


./o 

where  a  is  the  thickness  of  the  fluorescent  solution.     Hence 

— n^ 

i  —  e~^  Pi 

t  =  k  .  — - —  and  k  =  -      _ftll 

While  the  i  in  this  expression  represents  the  light  reaching  one  infini- 
tesimal portion  of  the  slit,  it  is  clear  that  the  total  light  reaching  the  slit 
is  proportional  to  i.  It  is  also  evident  that  k  is  proportional  to  the  total 
amount  of  light,  /,  of  the  wave-length  considered,  that  is  emitted  by  the 
fluorescent  substance  per  unit  volume.  Hence 


The  curve  showing  the  relation  between/  and  X  is  the  "typical  fluorescence 
spectrum." 

To  investigate  the  effect  of  concentration  upon  the  position  of  the 
fluorescence  band  observations  were  made  in  six  concentrations.  Table  6 
gives  values  of  observed  fluorescence  before  any  correction  is  made  for 
absorption.  Table  8,  graphically  represented  in  Fig.  31,  gives  the  same 
sets  of  observations  so  reduced  in  scale  as  to  be  comparable  in  intensity 
with  each  other.  This  reduction  was  made  by  multiplying  the  data  for 
each  concentration  by  such  a  factor  as  to  give  wave-length  0.589  /JL  the 
same  value  as  that  of  the  observed  fluorescence  of  the  corresponding  con- 
centration in  Table  9. 


ABSORBING   POWER   AND , FLUORESCENCE    OF   RESORUFIN. 
TABLE  6. 


35 


Observed  fluorescence  intensity  before  correction  is  made  for  absorption.  Meas- 
urements made  according  to  Method  I;  fluorescence  excitation  extending  over 
whole  face  of  cell. 


X 

J 

J 

iV 

:,'* 

«'< 

lh 

0.524^ 

'•77       2 

2.  I 

2 

2.08       2.4 

•542 

1.86       2.1 

2.2 

2                  2.13          2.41 

.562 

3.46       4.67 

5.6l          7.66          7.43 

7.27 

.589 

30.3       46.4 

56.6          34.9 

23.2 

16.2 

.614 

79-5       95-5 

95-3 

34  -o 

17.63 

it.  6 

.646 

86.6       76.3 

66.3 

17.7 

8.9 

6.88 

.69 

59  3 

42.4 

46.2 

12.7 

7.28 

TABLE  7. 

Observed  fluorescence  intensity  reduced  to  such  a  scale  as  to  make  intensity  of 
wave-length  o.  589  M  the  same  as  observed  fluorescence  in  Table  9. 


x           1           i 

,'« 

6 

8*4 

,ii 

0.524/1       5.38    ;     4.08 

3   '9 

3  32 

3.58 

3.12 

.542         5.65        4.28 

3-34 

3-32 

3.66 

3-23 

.562       10.5          9.53 

8-53 

12.7 

12.8 

9-73 

.589       92.2        94.6 

86.1 

57  9 

40.4         21.8 

.614       241  .6         195 

'45 

56.5 

30.3 

15-6 

.646       263             155 

101 

29.4 

15.3        9.22 

.69         180              86.5 

70.3 

21  .0 

12.5 

TABLE  8. 

Corrected  fluorescence.     Value  of  /  calculated  from  Table  6  according  to 
formula /  =  /£/(  i  —  e-0«). 


X 

i        I 

1*8 

31, 

* 

rh 

0.524/1 

2.84       1.97 

1.15 

O.67 

0.53         0.52 

-542 

444         3-o6 

'•74 

0.86 

0.632 

0.566 

.562 

1  1  .0 

9.09 

5-73       4  20 

2.56         1.85 

.589 

60.4 

56.6        37.2 

13.62 

6.35     ;     3.66 

.614 

33  3 

29.8 

23-5 

7.27 

3.45            2.20 

.646 

19.3 

"5-5 

12.8 

3  34 

!.65 

I  .22 

TABLE  9. 
Intensity  of  fluorescence  of  different  concentrations  at  wave-length  o.  589  /*. 


1 

J 

i 

A 

n 

ilf 

Observed  fluorescence  .        75-5 

| 
92.17,      94.64 

86.06 

57  95 

40.01 

21.68 

Corrected  fluorescence, 

t.  e.,  value  of  k  as  ob- 

tained from  formula  . 

206 

184             115           54.8 

22.5 

II  .0 

4.86 

!                        ! 

1 

STUDIES   IN    LUMINESCENCE. 


The  curves  in  Fig.  31  show  the  observed  values  of  /  as  a  function  of  X 
for  six  different  concentrations  of  the  solution.  The  shift  in  the  maximum 
caused  by  a  change  in  concentration  is  here  well  marked. 

Table  8  gives  the  result  of  correcting  values  in  Table  6  for  absorption 
according  to  the  above  formula,  the  value  of  /,  which  is  proportional  to 
actual  fluorescence  intensity  (see  page  34),  being  found  in  each  case.  These 


'250 


CJ200 


0.55>J  0.60.U 

Fig-  32. 


0.65>J 


Observed  fluorescence  intensity,  showing  shift  of  maxi- 
mum, with  change  in  concentration. 


Fluorescence  curves  corrected  for 
absorption. 


values  are  plotted  in  Fig.  32  according  to  an  arbitrary  vertical  scale  which 
makes  the  height  of  each  curve  approximately  proportional  to  intensity 
of  fluorescence  for  the  corresponding  concentration.  It  will  be  observed 
that  these  curves  are  similar  in  form,  the  maximum  wave-length  being 
approximately  the  same,  about  0.595  ju. 

For  purposes  of  comparison  a  series  of  measurements  was  made  to 
determine  the  relative  intensity  of  different  concentrations  at  wave-length 
0.589  /z,  keeping  all  conditions  constant.  Table  9  shows  the  results.  In 
Fig.  33  the  curve  marked  O  gives  the  actual  observed  fluorescence.  The 

200 — "^  c 


ISO 


100 


Si  - 

Fig.  33. — Concentration  and  fluorescence. 
Intensity  of  observed  and  corrected  fluorescence  for  different  concentrations  at  wave  length  0.589  M. 


ABSORBING  POWER  AND  FLUORESCENCE  OF  RESORUFIN. 


37 


"corrected"  curve,  C,  has  for  ordinates  the  value  of/  computed  from  the 
formula  (page  34)  and  shows  the  relative  intensity  of  actual  fluorescence  in 
different  concentrations. 

The  following  method  of  obtaining  the  corrected  fluorescence  curves 
was  used  as  a  check  upon  the  first,  the  final  results  being  obtained  by  a 
graphical  rather  than  a  mathematical  procedure. 

The  apparatus  was  set  up  as  before,  except  that  the  light  exciting  fluo- 
rescence, instead  of  striking  the  whole  face  of  the  glass,  was  allowed  to 
strike  only  a  vertical  section  i  mm.  in  width.  The  side  of  the  cell  next  to 
the  exciting  light  S'  was  covered  with  a  black  paper  screen  having  a  vertical 
opening  i  mm.  wide,  through  which  light  was  admitted.  This  opening 
was  set  at  different  distances  from  the  end  of  the  cell  and  the  fluorescence 
intensity  was  measured  for  three  or  four  different  positions.  In  order 
to  make  possible  accurate  measurements  of  the  distance  from  the  edge 
of  the  glass,  a  scale  was  etched 
upon  the  face  of  the  cell  just  above  I  so 
the  level  of  the  collimator  slit 

The  effect  of  absorption  upon 
the  position  of  the  maximum  of 
the  fluorescence  spectrum  is  well 
shown  in  Fig.  34,  in  which  the 
three  curves  were  taken  with  the 
opening  at  different  distances 
from  the  face  of  the  cell. 

In  order  to  correct  for  absorp- 
tion, curves  similar  to  those 
shown  in  Figs.  35  and  36  were 
plotted  for  each  solution.  These 
curves  show  the  variation  in  the 
intensity  of  the  fluorescent  light 
with  the  thickness  of  the  liquid 
through  which  the  light  passes. 
From  the  intercept  of  one  of  these 
curves  upon  the  y  axis  the  intensity 
of  fluorescence  can  be  found  when 
the  thickness  becomes  zero.  In 

such  a  case  there  is  no  absorption.  The  error  of  extrapolation  was  reduced 
to  a  minimum  by  taking  one  point  very  close  to  the  edge  of  the  glass. 

The  corrected  values  of  fluorescence  intensity  thus  obtained  were  used 
in  plotting  the  accompanying  fluorescence  curve  in  Fig.  36.  The  same 
procedure  was  followed  in  the  case  of  each  solution. 

The  observations  and  results  obtained  by  this  graphical  method  are 
given  in  Table  10.  The  corrected  fluorescence  spectra  obtained  in  this 
way  are  plotted  together  in  Fig.  37,  the  vertical  scale  being  such  that 
values  for  wave-length  0.589  /JL  are  the  same  as  the  values  for  the  corre- 
sponding concentration  in  Fig.  32. 

A  comparison  of  the  curves  of  Figs.  32  and  37  shows  that  the  results 
obtained  by  the  two  methods  are  the  same.  The  typical  fluorescence 
curves  are  all  similar  in  form  and  the  position  of  maximum  fluorescence 


0.55,0. 


o.ecyj. 
Fig.  34- 


Observed  fluorescence  produced  by  light  from  i  mm. 
section  oassing  throueh  different  thicknesses  of  solu- 
tion. Thickness  of  Di  =  i  mm.;  of  DJ=I  cm.;  of 

/>!  =2  cm. 


STUDIES  IN   LUMINESCENCE. 


(about  0.595  M)  remains  constant  for  all  dilutions.    The  shift  in  the  observed 
fluoresence  maximum  is  therefore  due  entirely  to  absorption. 

This  fixed  position  of  maximum  fluorescence  seems  to  be  consistent 
with  the  theory  of  ionization.  According  to  Buckingham,  ions  may  have 
some  of  the  same  optical  properties  as  other  substances  and  fluorescence 


Fig.  35- 

Variation  in  intensity  of  observed  fluorescence 
from  i  mm.  section  transmitted  through 
different  thicknesses  of  solution. 


I3U 

;00 
50 

r 

\ 

/ 

\ 

^ 

i 

0.55yU.            0.60JU,             0.6SJJ 

Fig.  36. 

Fluorescence  corrected  for  absorption  by 
graphical  method.     Concentration  =  }. 


is  one  of  these  properties.  If  only  the  ions  fluoresce  the  position  of  maxi- 
mum fluorescence  would  evidently  remain  constant  whether  the  material 
of  the  solution  were  all  ionized  or  not. 

This  fixed  position  of  maximum  fluorescence  is  also  consistent  with  the 
theory  suggested  in  Chapter  I,1  that  fluo- 
rescence is  caused  by  an  unusual  kind  of 
dissociation  similar  to  that  produced  in  a  gas 
by  the  action  of  Roentgen  rays.  That  part 
of  the  solution  which,  during  the  process  of 
change,  produces  fluorescence  does  not  give 
evidence  of  any  effect  due  to  material  not 
dissociated  beyond  that  of  absorption. 

SUMMARY. 

The  results  of  the  above  investigation  upon 
resorufin,  as  a  typical  fluorescent  substance, 
taken  in  connection  with  the  work  of  B. 
Walter  and  with  the  experiments  described 
in  Chapter  I,  seem  to  establish  the  truth  of 
the  following  statements: 

1.  Fluorescent    solutions     are     optically 
perfect  substances,/,  e.,  they  obey  Lambert's 
law. 

2.  Beer's   law,   i.   c.,  that  increase  of  concentration   is  equivalent   to 
increase  in  thickness,  is  true  for  dilute  but  not  for  concentrated  solutions. 

3.  A  change  in  the  concentration  of  a  fluorescent  solution  has  no  effect 
upon  the  typical  fluorescence  spectrum. 

'Nichols  and  Merritt,  Physical  Review,  xix.  No.  i,  1904;  xxn,  No.  5,  1906. 


0.55JU  0.60JJ 

Fig.  37- 


0.65,U 


Fluorescence  curves  corrected  for  ab- 
sorption by  graphical  method .  Ver- 
tical scale  same  as  for  Fig.  32. 


ABSORBING    POWER   AND   FLUORESCENCE    OF   RESORUFIN. 
TABLE  10. 


39 


Observed  fluorescence  of  section  i  mm.  wide  from  which  light  is  transmitted  through 
different  thickness  of  solution.  The  column  marked  o  thickness  gives  fluorescence 
intensity  corrected  for  absorption  by  graphical  method,  i.  e.,  by  extrapolation 
from  curves  similar  to  those  in  Fig.  35. 


Concentration  J. 

Concentration  J. 

1.5  mm. 

5  mm. 

i  cm. 

0 

1.5  mm. 

5  mm. 

i  cm. 

0 

O    524u          I    "75 

i  .  i 

1    c 

.542        2.52         1.6 

1  .2 

2 

2.83 

2.  1 

'•3 

3 

.562        14.3           3 

1.95 

12.5 

9.07 

4.1 

2.32 

13.8 

.589       66.2         41.5 

29.6 

114 

7'-4 

45-9 

21.4 

103 

.614       94.5         70.7 

6l.2 

103 

88.1 

73-7 

64.6           95 

.  646       42  .  4 

42 

42.5 

43 

5'-4 

48.7 

54-3           52-3 

Concentration  |. 

Concentration  T'g. 

i  .  5  mm  . 

5  mm. 

i  cm.          Oil  mm. 

i  cm. 

2  cm. 

0 

o.  52411 

2.65 

2 

1    7t 

2     5 

2.83 

3  .  5 

.542 

39 

2 

2  .  66         4             5  .  92 

2.97 

2.83 

8 

.562 

34   1  5 

12.  I 

7.52       50           34.8 

8.62 

5.56 

42 

.589 

96             69.7 

49.5        109         132 

75-9 

55-4 

144 

.6 

102             79.5 

63.3        117 

.614 

79.72       68.4 

56.5            90            121 

I  10 

99.6 

124 

.646 

34  7         33-6 

31             36           67  .  3 

72.7 

60.3 

70 

Concentration  K\. 

Concentration  ,J8. 

X 

1.5  mm. 

5  mm. 

i  cm. 

o 

1.5  mm. 

5  mm. 

i  cm. 

o 

0.508^ 

3  47 

3.81 

3 

4 

.524 

4.83 

3-75 

3  75 

4 

4-'7 

4  47 

3-45 

4-5 

.542 

6.16 

49 

4.23 

7 

6.52 

6.1 

5-33 

6.8 

.562 

62 

38.5 

25-5 

76 

25.2 

23-7 

'7 

26 

.589 

146 

138 

125 

150 

49-2 

49-5 

47-4 

49-5 

.614 

i  10   ;  105 

96.1 

i  '3 

39-7 

37-7 

38.3 

39-5 

.646 

58.1 

50.2 

3»-7 

60 

27.8 

25 

27-5 

28 

CHAPTER  III. 


THE  LUMINESCENCE  OF  SIDOT  BLENDE.1 

Among  the  substances  whose  luminescence  is  sufficiently  bright  for 
spectrophotometric  study,  Sidot  blende  or  phosphorescent  zinc  sulphide 
seems  especially  well  suited  to  bring  out  the  relationships  that  doubtless 
exist  between  different  types  of  luminescence ;  for  not  only  is  this  substance 
excited  to  luminescence  by  all  known  exciting  agents — light,  Roentgen 
rays,  radium  rays,  cathode  rays,  etc. — but  the  stimulating  effect  of  heat 
and  the  property  possessed  by  the  red  and  infra-red  rays  of  suppressing 
phosphorescence  are  exhibited  in  unusual  degree.  It  is  for  this  reason  that 
we  have  chosen  this  substance  for  spectrophotometric  study. 

The  material  used  in  these  experiments  was  in  the  form  of  a  screen,  of 
the  kind  frequently  used  in  exhibiting  the  properties  of  radium  and  for 
numerous  experiments  in  which  it  is  desirable  to  have  a  fluorescent  sub- 
stance in  convenient  form.  The  experimental  methods  were  similar  to 


Fig.  38- 

The  uppercurve  shows  the  luminescence 
spectrum  of  Sidot  blende  during  ex- 
citation by  Roentgen  rays.  The  lower 
curve  shows  the  phosphorescence  5 
seconds  after  excitation. 


20 


0.50 


054 


0.58 


O.S2JJ, 


those  described  in  Chapters  I  and  II,  a  Lummer-Brodhun  spectrophoto- 
meter,  with  an  acetylene  flame  as  a  comparison  source,  being  used  to  measure 
the  intensity  in  different  parts  of  the  luminescence  spectrum.  In  certain 
portions  of  the  work  special  devices  were  required  which  will  be  described 
in  their  proper  place. 

LUMINESCENCE  EXCITED  BY  ROENTGEN  RAYS. 

The  screen  was  placed  in  front  of  the  collimator  slit  of  the  spectrophoto- 
meter  at  a  distance  of  only  a  few  centimeters,  while  the  Queen  self-regulating 
tube  used  for  excitation  was  about  20  cm.  behind  the  screen.  Roentgen 
rays  of  moderate  "hardness"  were  used.  No  systematic  experiments  to 
determine  the  effect  of  varying  hardness  upon  the  form  of  the  luminescence 
spectrum  have  yet  been  made,  but  a  few  preliminary  tests  indicate  that 
the  effect  is  not  great.  Corrections  due  to  fluorescence  excited  in  the 
glass  of  the  spectrophotometer,  which  we  had  at  first  thought  would  be 
necessary,  were  found  to  be  negligible. 

The  upper  curve  in  Fig.  38  shows  the  luminescence  spectrum  observed 
during  excitation,  while  the  lower  curve  shows  the  distribution  of  intensity 


•An  account  of  the  experiments  described  in  this  chapter  was  presented  to  the  American  Physical  Society 
at  the  Philadelphia  meeting  December  30, 1904.     Physical  Review,  xxi.p.  247;  xxii.  p.  279;  XXHI,  p.  37. 

41 


STUDIES  IN   LUMINESCENCE. 


in  the  phosphorescence  spectrum,  observed  5  seconds  after  excitation  had 
ceased.  Owing  to  the  weakness  of  the  phosphorescence  excited  by  Roentgen 
rays,  the  latter  curve  was  determined  with  some  difficulty,  the  readings  at 
the  red  end  of  the  spectrum  being  especially  uncertain.  The  indications 
of  a  second  maximum  in  the  phosphorescence  spectrum  at  0.62  fj,  or  beyond 
are  therefore  not  to  be  regarded  as  entirely  trustworthy. 

PHOTO-LUMINESCENCE   DURING   EXCITATION. 

In  the  experiments  upon  photo-luminescence  during  excitation  the  carbon 
arc  was  first  used  as  an  exciting  source.  The  arrangement  of  apparatus 
is  shown  in  Fig.  39.  The  zinc  sulphide  screen  was  placed  in  a  light-tight 
box  B,  and  the  exciting  light  from  an  arc  A ,  after  dispersion  by  the  prism  P, 
entered  the  box  through  an  opening  at  the  left.  A  second  opening  in  B 
allowed  the  fluorescence  light  to  enter  the  slit  of  the  Lummer-Brodhun 
spectrophotometer  Si.  The  comparison  source  was  an  acetylene  flame  F, 


zoo 


Fig.  39- 


80 


40 


.440        .480       .520/i      .560        .600 

Fig.  40. 

Luminescence  spectrum  of  Sidot  blende 
during  excitation  by  the  violet  bands 
in  the  arc. 


whose  rays  were  reflected  into  the  comparison  slit  by  the  block  of  magne- 
sium carbonate,  M.  The  second  spectrophotometer  S2  and  the  shutter  C 
were  used  in  later  experiments  on  phosphorescence,  but  not  in  the  experi- 
ments now  considered. 

Luminescence  was  excited  only  by  the  rays  at  the  violet  end  of  the  arc 
spectrum,  and  especially  in  the  region  of  the  violet  bands.  It  is  to  be  noted 
that  on  account  of  the  absorption  of  the  glass  prism  and  lenses  the  spectrum 
extended  only  a  short  distance  into  the  ultra-violet. 

Observations  taken  to  determine  the  luminescence  spectrum  when  the 
zinc  sulphide  screen  was  excited  by  the  violet  bands  of  the  arc  are  plotted 
in  Fig.  40.  Owing  to  the  impurity  of  the  exciting  spectrum  a  certain 
amount  of  blue  and  green  light  reached  the  screen  even  when  the  attempt 
was  made  to  excite  by  violet  only.  Since  this  stray  light  was  in  part 
reflected  into  the  slit  of  the  spectrophotometer  with  the  luminescence  light 
of  the  same  color,  it  was  necessary  to  make  a  correction  to  remove  the  error 
due  to  this  cause.  The  correction  was  determined  by  replacing  the  zinc 


THE    LUMINESCENCE    OF   SIDOT   BLENDE.  43 

sulphide  screen  by  a  block  of  magnesium  carbonate,  whose  surface  was  of 
approximately  the  same  color  and  roughness  as  that  of  the  fluorescent 
screen,  and  by  measuring  the  light  received  from  this  block  at  different 
points  throughout  the  spectrum.  The  results  of  such  a  series  of  measure- 
ments is  shown  in  the  lower  broken  line  in  the  figure.  It  will  be  noticed 
that  the  correction  is  inappreciable  for  the  longer  wave-lengths,  but  be- 
comes important  in  the  violet.  The  upper  broken  line  in  Fig.  40  shows 
the  intensity  of  the  light  reaching  the  spectrophotometer  from  the  zinc 
sulphide  screen,  being  the  combined  effect  of  luminescence  and  reflected 
light;  and  the  heavy  curve,  obtained  by  subtracting  the  ordinates  of  the 
lower  curve  from  those  of  the  upper,  shows  the  corrected  luminescence 
spectrum  during  excitation. 

The  curves  in  Fig.  40,  like  those  shown  in  the  previous  chapters,  are 
expressed  in  terms  of  the  acetylene  flame  as  a  standard.  In  other  words, 
each  ordinate  represents  the  ratio  of  the  intensity  of  the  luminescence  light, 
of  the  particular  wave-length  considered,  to  the  intensity  of  the  light  of  the 
same  wave-length  from  the  acetylene  flame;  the  curves  are  not  energy 
curves.1 

When  the  screen  was  excited  by  the  arc  as  described  above  no  trace 
of  the  violet  luminescence,  which  had  been  so  prominent  with  Roentgen- 
ray  excitation,  could  be  observed.  It  seemed  not  unlikely,  however,  that 
the  absence  of  the  violet  band  was  due  to  the  fact  that  the  ultra-violet  rays 
suitable  for  exciting  it  had  been  removed  from  the  light  from  the  arc  by  the 
glass  prism  and  lenses  of  the  dispersing  system.  We  therefore  rearranged 
the  apparatus  so  as  to  use  a  quartz  prism  and  quartz  lenses,  while  a  spark 
between  metal  terminals  was  substituted  for  the  arc.  Since  the  excitation 
in  the  region  studied  was  solely  by  ultra-violet  rays  that  were  incapable 
of  passing  through  glass,  it  was  a  simple  matter  to  test  for  errors  due  an 
impure  spectrum,  or  to  any  other  source  of  stray  light,  by  inserting  a  piece 
of  glass  in  the  path  of  the  exciting  rays.  This  test  showed  that  stray  light 
was  in  no  case  present  in  appreciable  amount. 

This  ultra-violet  excitation  developed  strong  fluorescence  in  the  extreme 
violet,  similar  in  color  to  that  produced  by  Roentgen  rays.  But  the  green 
band  that  had  been  excited  by  the  visible  rays  of  the  arc  was  relatively 
very  weak.  In  spite  of  its  faintness,  however,  the  presence  of  the  green 
band  could  be  readily  detected,  for  while  the  violet  luminescence  died  out 
almost  immediately  when  excitation  ceased,  the  green  persisted  as  a  slowly 
decaying  phosphorescence  observable  for  several  minutes. 

The  similarity  between  the  effects  of  ultra-violet  light  and  those  of 
Roentgen  rays  as  exciting  sources  is  worthy  of  note.  In  each  case  the  chief 
luminescence  is  in  the  extreme  violet,  and  is  of  short  duration.  But  in 
each  case  also  this  is  accompanied  by  luminescence  in  the  green,  which  is 
relatively  faint  but  of  long  duration.  As  is  illustrated  in  numerous  other 
cases,  the  Roentgen  rays,  when  comparable  at  all  with  rays  of  light,  are 
rather  to  be  compared  with  ultra-violet  light  than  with  the  rays  of  the 
visible  spectrum. 

The  agreement  between  the  luminescence  spectrum  excited  by  Roentgen 
rays  and  that  observed  in  the  case  of  ultra-violet  excitation  is  not  exact. 

'For  a  description  of  a  method  of  reducing  such  curves  giving  the  distribution  of  energy  in  fluorescence 
spectra  see  Nichols  and  Merritt,  Physical  Review,  xxx,  p.  328;  also  Chapter  XII  of  this  memoir. 


44 


STUDIES  IN   LUMINESCENCE. 


We  are  of  the  opinion  that  the  difference  is  due  to  the  fact  that  the  violet 
luminescence  does  not  consist  of  one  band  only,  but  of  at  least  two,  which 
are  excited  by  Roentgen  rays  and  ultra-violet  light  in  different  relative 
intensity.  The  matter  will  be  made  clear  by  reference  to  Fig.  41.  Curve  / 
shows  the  luminescence  spectrum  when  ultra-violet  rays  from  the  iron  spark 
were  used  for  excitation.  The  rays  used  were  those  giving  most  intense 
luminescence,  and  did  not  lie  much  beyond  the  edge  of  the  visible  spectrum. 
The  shape  of  this  curve  suggests  that  it  is  made  up  of  two  overlapping 
bands,  one  having  a  maximum  not  far  from  0.48  n,  while  the  other  has  its 
maximum  near  0.42  p.  In  curve  //  the  exciting  light  was  at  the  extreme 
ultra  end  of  the  iron  spectrum,  and  in  curve  ///  the  magnesium  lines  near 
O-33  M  were  used  in  excitation.  In  these  curves  also  there  is  every  indication 
that  the  spectrum  consists  of  at  least  two  bands. 

Unfortunately  we  have  not  been  able  to  separate  the  bands  more 

completely.  It  is  to  be 
remembered  that  the  re- 
solving power  of  a  spec- 
trophotometer  is  at  the 
best  small,  and  that  in 
experiments  of  the  kind 
here  described  the  small 
intensity  of  the  light 
studied  prohibits  the  use 
of  a  narrow  slit.  Lumin- 
escence bands  may  some- 
times be  separated  by  a 
proper  choice  of  the  excit- 
ing light.  In  the  case 
of  Sidot  blende  the  green 
band  is  very  readily  ob- 
tained alone  by  using  the 
carbon  bands  of  the  arc 
for  excitation.  But  the 
two  violet  bands  in  the 
luminescence  of  this  sub- 
stance are  so  close  to- 
gether that  their  regions  of  excitation  appear  to  overlap  throughout  nearly 
their  whole  extent.  The  most  that  we  were  able  to  do  was  to  show  that 
ultra-violet  rays  of  different  wave-length  produced  different  relative  inten- 
sities in  the  two  bands,  as  illustrated  by  curves  /  and  ///  of  Fig  41. 

The  broken  line,  curve  IV  of  Fig.  41,  shows  the  luminescence  spectrum 
excited  by  Roentgen  rays.  This  is  the  same  curve  that  appears  in  Fig.  38, 
and  is  introduced  in  Fig.  41  to  facilitate  the  comparison  of  the  effects  of 
the  ultra-violet  light  and  Roentgen  rays.  It  will  be  observed  that  each  of 
the  curves  in  Fig.  41  might  well  result  from  the  superposition  of  three  bands, 
whose  maxima  lie  approximately  ato.42  /u,  0.48  ju  and  0.5 1  /j..  In  the  Roent- 
gen-ray luminescence  the  green  band  is  relatively  stronger  than  in  the  case 
of  excitation  by  ultra-violet  light,  while  the  band  at  0.48  ju  appears  to  be 
entirely  absent.  In  the  luminescence  produced  by  the  magnesium  spark 


ao 


0.46 


0.50 

Fig.  41. 


0.54/i 


Fluorescence  spectrum  of  Sidot  blende  when  excited  by  the  ultra- 
violet rays  of  the  iron  spark  (curves  /  and  //),  by  the  ultra- 
violet of  the  magnesium  spark  (curve  ///),  and  by  the  Roentgen 
rays  (curve  IV). 


THE    LUMINESCENCE   OF   SIDOT   BLENDE.  45 

(curve  ///)  the  green  band,  while  still  observable,  is  not  so  strong  as  in  curve 
IV,  while  the  band  at  0.48/1  is  readily  detected.  Curves  7  and  //  (iron 
spark)  show  scarcely  any  trace  of  the  green  band,  but  the  bands  at  0.48/1  and 
at  0.42/1  are  well  marked. 

FAILURE  OF  STOKES'S  LAW. 

As  already  stated,  the  green  band  is  most  brilliantly  excited  by  the 
violet,  although  rays  from  all  parts  of  the  ultra-violet  spectrum  are  also 
capable  of  producing  a  considerable  effect.  We  thus  see  that  there  is  the 
same  general  relation  between  the  position  of  the  luminescence  spectrum 
and  that  of  the  exciting  light  as  in  the  case  of  fluorescence.  In  all  the  cases 
of  fluorescence  thus  far  studied  we  have  found  that  the  two  spectral  regions 
overlap.  It  therefore  seemed  interesting  to  see  whether  the  same  is  true  in 
the  present  case. 

To  settle  this  point  it  was  necessary  to  use  in  excitation  a  far  purer 
spectrum  than  that  employed  in  our  other  experiments  with  Sidot  blende. 
The  rather  crude  dispersing  system  shown  in  Fig.  39  was  therefore  replaced 
by  a  large  spectrometer.  A  spectrum  of  the  arc  was  thrown  on  the  colli- 
mator  slit  by  the  aid  of  a  prism  and  single  lens,  and  after  a  second  dis- 
persion in  the  spectrometer,  light  reached  the  phosphorescent  screen  as  a 
sharply  focused  band.  Using  a  shutter  described  below  so  as  to  observe 
the  phosphorescence  immediately  after  the  exciting  light  was  cut  off,  and 
making  observations  with  the  unaided  eye  instead  of  with  the  spectrophoto- 
meter,  it  was  found  that  phosphorescence  was  unquestionably  present  for 
exciting  light  lying  between  0.470/1  and  0.497/1.  Since  the  phosphores- 
cence spectrum  can  readily  be  followed  beyond  0.46  /i  there  appears  to 
be  the  same  violation  of  Stokes's  law  in  the  phosphorescence  of  Sidot 
blende  that  we  have  previously  found  in  the  case  of  fluorescence. 

THE  PHOSPHORESCENCE   SPECTRUM    DURING   DECAY. 

Although  the  phosphorescence  of  Sidot  blende  can  be  detected  in  a  dark 
room  for  several  hours  after  excitation  has  ceased,  it  remains  sufficiently 
bright  for  spectrophotometric  measurement  only  for  a  few  seconds.  In 
order  to  determine  the  law  of  decay  for  different  wave-lengths  and  especially 
to  determine  the  change,  if  any,  in  the  phosphorescence  spectrum  during 
decay,  the  following  procedure  was  followed : 

A  shutter,  shown  by  the  broken  line  C  in  Fig.  39,  and  sliding  upon  vertical 
guides,  was  attached  to  the  box  containing  the  phosphorescent  screen. 
When  this  shutter  was  raised  it  permitted  the  exciting  light  to  enter  the 
box,  as  in  the  experiments  just  described,  but  at  the  same  time  closed  the 
opening  in  front  of  the  collimator  slit.  When  the  shutter  was  dropped  it 
first  cut  off  the  exciting  light  and  then,  a  moment  later,  opened  the  window 
in  front  of  the  collimator.  The  observer  was  in  this  way  protected  from 
the  brilliant  luminescence  produced  during  excitation,  but  was  enabled  to 
observe  the  phosphorescence  immediately  after  excitation  had  ceased.  The 
comparison  slit  being  set  to  some  suitable  reading,  the  observer  recorded 
upon  a  chronograph  the  instant  when  the  two  parts  of  the  spectrophoto- 
meter  field  appeared  equally  bright.  By  means  of  a  suitable  electric  contact 
on  the  shutter  a  record  was  also  made  at  the  time  when  the  exciting  light 


46 


STUDIES  IN   LUMINESCENCE. 


was  cut  off.  In  this  way  the  time  required  for  the  phosphorescent  light 
to  fall  from  its  initial  value  to  any  given  final  value  could  be  conveniently 
measured. 

The  determination  of  the  instant  when  the  rapidly  changing  phospho- 
rescence became  equal  to  the  constant  comparison  light  would  at  first 
appear  to  be  a  difficult  observation,  and  one  not  capable  of  great  accuracy. 
As  a  matter  of  fact  these  observations  were  fully  as  reliable  as  ordinary  spec- 
trophotometric  settings,  and  at  these  low  intensities  far  less  trying  to  the  eye. 

The  most  serious  difficulty  encountered  in  these  measurements  arose 
from  the  unsteadiness  of  the  exciting  light.  When  constancy  is  essential 
the  arc,  even  under  the  best  of  conditions,  leaves  much  to  be  desired. 
After  numerous  unsuccessful  attempts  to  obtain  steady  conditions  we 
abandoned  all  special  efforts  to  keep  the  excitation  constant,  and  arranged 
to  make  observations  only  when  the  exciting  light,  either  by  adjustment 
or  by  accidental  fluctuation,  had  reached  a  certain  definite  intensity.  In 
order  to  accomplish  this  a  second  spectrophotometer  (£2  in  Fig.  39)  was 
used.  Light  from  the  luminescent  screen  was  thrown  into  the  collimator 
by  means  of  a  mirror  as  shown  in  the  figure.  The  comparison  light  came 
from  the  acetylene  flame  F  after  reflection  from  a  mirror  and  a  block  of 
magnesium  carbonate  not  shown  in  the  figure.  One  observer  at  the  eye- 
piece of  S2  waited  until  the  intensity  of  fluorescence  reached  such  a  value  as 
to  give  equality  of  the  two  fields,  and  then,  with  suitable  warning,  dropped 
the  shutter  C.  The  second  observer  at  Si  then  made  a  chronograph  record 
as  before  described.  Numerous  determinations,  often  twenty  or  more,  were 
made  in  this  way  for  each  point  of  the  curves  described  below. 

TABLE  11. 

The  observed  time,  in  seconds,  during  which  the  phosphorescence  fell  from 
its  initial  intensity  to  the  intensity  at  the  top  of  the  column. 


Wave-length. 

7  =  100. 

7=60. 

7=30. 

7=10. 

7  =  5. 

0.461^ 
.472 
.483 

o.oo 

AQl 

o.oo 

.456 

QC 

0-745 

1  63 

0.458 

0.99 
2.07 

a    1} 

7-1 

.512 

528 

.  502 

290 

1  .00 
o  682 

1.80 

1    4Q 

3.4. 

2  -Ol 

10.4 

535 

oo 

547 

0.223 

0.838 

2.55 

7-5 

(U 

o  oo 

568 

0.302 

1  .30 

C72 

^ 
o.oo 

CQ2 

0.713 

4-  ' 

A  preliminary  set  of  measurements  in  which  the  duration  of  excitation 
was  varied  from  3  seconds  to  30  seconds  showed  no  variation  in  the  time 
of  decay.  We  conclude  that  the  full  effect  of  the  exciting  light  is  produced 
in  less  than  3  seconds.1  In  the  subsequent  experiments  the  duration  of 
excitation  was  never  less  than  5  seconds. 

The  results  of  one  set  of  determinations  made  in  this  way  are  given  in 
Table  1 1  and  are  plotted  in  Fig.  42.  In  curve  7  the  width  of  the  comparison 

"Later  experiments  (see  Chapter  IV)  show  that  the  long-time  phosphorescence  of  Sidot  blende  is  very 
noticeably  influenced  by  a  change  in  the  duration  of  excitation. 


THE    LUMINESCENCE    OF    SIDOT   BLENDE. 


47 


slit  was  kept  at  100  divisions  of  its  micrometer  screw,  and  the  time  required 
for  the  phosphorescence  of  any  given  wave-length  to  fall  from  its  initial 
value  to  this  intensity  was  determined  as  described  above.  The  curve 
is  obtained  by  plotting  wave-lengths  as  abscissas  and  times  as  ordinates. 
In  curve  //  the  width  of  the  comparison  slit  was  60  divisions ;  in  curve  777, 
30  divisions;  and  in  curve  IV,  10  divisions.  Observations  made  at  5  divi- 
sions are  not  included  on  the  plot.  Points  lying  on  the  horizontal  axis  in 
Fig.  42  were  located  by  determining  by  trial  the  wave-length  for  which 
there  was  a  balance  in  the  spectrophotometer  the  instant  after  dropping  the 
shutter.  These  points  are  probably  somewhat  less  reliable  than  the  others. 
The  significance  of  these  results  is  better  shown  by  plotting  them  in  a 
different  way.  In  Fig.  43  wave-lengths  are  plotted  as  abscissas  as  before, 
but  intensities,  instead  of  times,  are  plotted  as  ordinates.  Each  curve  in 
this  figure  shows  the  distribution  of  intensity  in  the  phosphorescence  spec- 
trum at  some  definite  time  after  the  removal  of  the  exciting  light 


IV 

\ 


III 


\ 


.460       .480 


.500       .520       .540 

Fig.  42. 


.560       .580      .600 


Curves  showing  the  time  required  for  the  phosphorescence  to  fall  from 
its  initial  intensity  to  a  given  final  intensity.  The  final  intensity  is 
kept  constant  throughout  each  curve. 

No  curves  have  been  plotted  for  intervals  of  more  than  1.75  seconds 
after  excitation  had  ceased,  since  our  data  furnished  so  few  points  for  the 
later  curves  that  their  form  would  be  largely  a  matter  of  conjecture.1  As 
already  stated,  however,  a  curve  of  the  type  shown  in  Fig.  42  was  deter- 
mined for  an  intensity  of  5  divisions.  The  maximum  ordinate  of  this 
curve  was  10.1  seconds  and  occurred  at  0.506  n;  i.  e.,  at  the  same  wave- 
length as  in  the  case  of  the  curves  shown  in  42.  Comparison  with  Fig.  40 
shows  that  the  maximum  of  the  luminescence  spectrum  during  excitation 
also  occurs  at  this  wave-length.  In  the  case  of  the  green  band  of  Sidot 
blende  we  conclude  therefore  that  the  maximum  of  the  fluorescence  spectrum 
occurs  at  the  same  wave-length  as  that  of  the  phosphorescence  spectrum, 


'In  a  series  of  observations  of  the  kind  recorded  in  Fig.  42  the  number  of  points  that  could  be  located  was 
limited  both  by  the  difficulty  of  maintaining  constant  conditions  during  the  three  or  four  hours  of  observa- 
tion, and  by  the  endurance  of  the  observer's  eye. 


STUDIES  IN   LUMINESCENCE. 


100 


and  that  no  change  that  could  be  detected  by  our  apparatus  occurs  in  the 
position  of  this  maximum  for  10  seconds  after  excitation  has  ceased.1 

Fig.  43  also  shows  that  if  any  change  occurs  in  the  form  of  the  phospho- 
rescence spectrum  during  decay,  this  change  is  extremely  small.     In  other 

words,  the  different  wave-lengths 
of  the  phosphorescence  spectrum 
decay  at  the  same  rate.  Table  12 
will  show  to  what  extent  this  con- 
clusion is  justified  by  the  data. 
Each  column  of  this  table  refers  to 
the  wave-length  stated  at  the  top, 
and  in  it  are  tabulated  the  intensi- 
ties for  this  wave-length  at  different 
intervals  after  the  end  of  excitation, 
these  intensities  being  expressed  in 
terms  of  the  intensity  at  the  end  of 
1.75  seconds  as  unity.  If  the  phos- 
phorescence spectrum  remains  ab- 
solutely unaltered  during  decay, 
all  the  numbers  in  this  table  that 
lie  in  a  given  horizontal  line  should 
be  equal. 

The  numbers  in  Table  12  corre- 
sponding to  o.o  second  are  uncer- 
tain; first,  because  the  observations 
by  which  the  initial  phosphores- 
cence was  determined  were  them- 
selves especially  uncertain;  and  second,  because  the  steepness  of  the  curve 
(Fig.  43)  in  the  neighborhood  of  0.48  ^  and  0.54  n  would  cause  a  slight 
change  in  the  manner  of  plotting  to  produce  a  large  change  in  the  ordinates 
at  these  wave-lengths.  If  the  ratios  tabulated  for  o.o  second  are  left  out 
of  consideration,  the  agreement  between  the  remaining  ratios,  at  0.5  second 


.460 


.500  .540 

Fig-  43- 


.580// 


Phosphorescence    spectrum    of  Sidot  blende  at  <tif- 
erent  times  after  removal  of  the  exciting  light. 


TABLE  12. 

Intensity  of  phosphorescence  at  different  intervals  after  excitation 
ceased,  expressed  in  terms  of  the  intensity  at  the  end  of  1.75  sec. 


had 


0.48/1 

0.50/x 

0.52M       0.54M 

0.56*1 

o.o    sec. 

7.8? 

5.2 

4.8 

0.5    sec. 

3-5 

3-3 

3.1       3.0 

2.8 

i.o    sec. 

1.9 

2.0 

1.8          1.8 

1.9 

i  .75  sec. 

1  .0 

I.O 

I.O            I.O 

I  .0 

and  i  .o  second,  is  as  close  as  could  be  expected.  We  conclude  that,  although 
there  is  some  indication  of  more  rapid  decay  at  the  ultra  edge  of  the  phos- 
phorescence spectrum,  the  probability  is  that  all  parts  of  the  spectrum  decay 
at  the  same  rate,  and  that  the  form  of  the  phosphorescence  spectrum 
remains  unchanged. 

'Studies  of  the  phosphorescence  spectrum  of  Sidot  blende  by  a  photographic  method,  which  made  it 
possible  to  determine  the  position  of  the  maximum  at  various  times  up  to  90  seconds  after  excitation  had 
ceased  and  which  confirm  this  conclusion  and  extend  it,  will  be  described  in  Chapter  VIII. 


THE   LUMINESCENCE   OF   SIDOT  BLENDE-  49 

While  the  result  obtained  in  this  one  case  is  not  sufficient  to  establish  a 
general  law,  we  are  nevertheless  of  the  opinion  that  the  behavior  of  Sidot 
blende  is  typical,  and  that  in  no  case  of  phosphorescence  is  there  any  change 
in  the  form  of  a  band  during  decadence.  In  complex  cases  of  phosphores- 
cence we  do  not  imply  by  this  that  the  phosphorescence  spectrum  as  a  whole 
remains  unchanged  in  form,  but  rather  that  the  distribution  of  intensity 
in  each  band  is  unaltered.  If  the  phosphorescence  consists  of  several 
bands,  it  is  to  be  expected  that  the  different  bands  will  decay  at  different 
rates.  In  fact  Sidot  blende  itself  furnishes  an  extreme  illustration  of  this, 
for  the  violet  bands  die  out  in  one  or  two  tenths  of  a  second,  while  the  green 
band  persists  for  hours. 

Numerous  cases  in  which  the  color  of  a  phosphorescent  substance  seems 
to  change  as  the  phosphorescence  dies  out  at  first  appear  to  contradict 
this  view.  We  think,  however,  that  all  cases  of  this  kind  may  be  shown 
to  belong  to  one  of  the  two  following  classes : 

1.  Cases  of  real  color  change;  for  example,  anisic  acid  at  low  tempera- 
tures, where  the  phosphorescence  changes  from  blue  to  greenish  yellow.1 
Such  cases  are  probably  due  to  the  presence  in  the  luminescence  spectrum 
of  several  phosphorescence  bands,  which  decay  at  different  rates.     In  the 
case  of  anisic  acid  the  results  would  be  explained  by  the  presence  of  a 
brilliant  but  rapidly  decaying  band  in  the  blue,  and  a  persistent  band  of 
smaller  initial  intensity  in  the  yellow. 

2.  Cases  where  the  apparent  change  in  color  is  due  to  the  fact  that  the 
color  sense  in  the  eye  is  either  weak  or  entirely  absent  at  low  intensities. 
At  very  low  intensities  all  colors  appear  to  the  eye  as  gray.     However 
brilliant  may  be  the  color  of  the  phosphorescence  light  initially,  it  loses  this 
color  and  changes  to  a  gray  or  faint  white  as  it  becomes  fainter.     But 
such  a  change  as  this  is  in  the  retina,  and  does  not  indicate  any  change 
in  the  phosphorescence  spectrum. 

It  is  interesting  to  note  that  attention  was  called  to  both  of  these  causes 
of  change  of  color  during  decay  by  the  elder  Becquerel.2 

The  data  of  Table  1 1  might  also  be  used  to  study  the  law  of  decay  of 
phosphorescence,  i.  e.,  the  relation  between  intensity  and  time.  More 
reliable  results  are  obtained,  however,  by  studying  one  wave-length  at  a 
time,  since  the  data  necessary  for  plotting  a  decay  curve  may  in  this  case 
be  obtained  more  quickly  and  are,  therefore,  less  liable  to  error  due  to 
fluctuating  conditions.  Measurements  of  phosphorescence  spectra  during 
decadence,  in  the  case  of  other  substances,  are  described  in  Chapters  IV 
and  VII. 


'Nichols  and  Merritt,  Physical  Review,  18,  p.  355,  1904. 

2Ed.  Becquerel.  Comptes  Rendus,  49,  p.  27, 1859;  Ann.de  Chimie  et  de  Physique,  series  4,  62,  p.  20,1861. 


CHAPTER  IV. 

THE  DECAY  OF  PHOSPHORESCENCE  IN  SIDOT  BLENDE  AND 
CERTAIN  OTHER  SUBSTANCES.1 

The  decay  of  phosphorescence  was  first  studied  by  B.  Becquerel.2  In 
the  case  of  short-duration  phosphorescence  the  phosphoroscope  was  used 
for  this  purpose  and  the  intensity  of  phosphorescence  was  measured  for 
different  speeds  of  the  rotating  disk.  Becquerel  regarded  it  as  probable 
that  the  law  of  decay  was  of  the  form 

I=he~al  (i) 

and  found  that  the  observations  were  in  fact  fairly  well  represented  by 
an  exponential  expression.  In  discussing  these  observations,  however, 
Becquerel  tacitly  assumed  that  no  appreciable  time  was  required  for  the 
exciting  rays  to  produce  their  full  effect.  Later  investigations  have  shown 
that  this  assumption  is  not  justified.  Since  a  change  in  the  speed  of  the 
phosphoroscope  altered  not  only  the  time  that  elapsed  between  excitation 
and  observation,  but  also  the  duration  of  exposure,  it  is  probable,  therefore, 
that  the  initial  excitation  was  less  at  high  speeds.  Attention  was  directed 
to  this  point  by  E-  Wiedemann  and  later  by  H.  Becquerel,  who  states  that 
a  recomputation  of  the  data  shows  a  less  satisfactory  agreement  with  the 
exponential  law  than  at  first  appeared. 

For  the  long-time  phosphorescence  of  the  phosphorescent  sulphides  E. 
Becquerel  proposed  an  empirical  expression  of  the  form 

Im(c+t)=cI0m  (2} 

For  each  of  the  seven  substances  tested  this  expression  was  found  to  show 
a  fairly  good  agreement  with  the  experimental  results  throughout  a  con- 
siderable range.  In  one  case,  namely,  that  of  a  calcium  sulphide  prepara- 
tion giving  an  orange-red  phosphorescence,  the  expression  represented 
the  observations  with  considerable  accuracy  throughout  the  whole  range, 
the  value  of  m  being  0.5.  But  in  most  cases  it  was  not  possible  to  find 
values  of  m  and  c  which  would  make  the  formula  fit  the  experimental  data 
for  the  whole  time  of  decay.  The  values  of  m  that  suited  the  observations 
best  lay  between  0.5,  for  the  calcium  sulphide  just  mentioned,  and  0.806 
for  another  calcium  sulphide  having  an  orange-yellow  phosphorescence. 

The  same  empirical  formula  has  since  been  very  generally  used,  among 
others  by  Darwin3  in  1881,  and  Ch.  Henry4  in  1892.  The  former  worked 
with  Balmain's  paint  and  found  the  best  value  for  m  to  be  0.86.  It  can 
not  be  said,  however,  that  the  experimental  results  were  represented  very 
accurately  by  the  formula.  The  substance  used  by  Henry  was  Sidot  blende, 

'An  account  of  the  work  described  in  this  chapter  was  presented  to  the  American  Physical  Society  at  the 
meeting  held  on  Oct.  28,  1905.  Physical  Review,  xxir,  p.  279. 

2Becquerel,  "La  Lumiere."  See  also  Comptes  Rendus,  51,  p.  921,  1860,  and  Annales  de  Chimie  et  de 
Physique,  series  4,  62,  p.  5. 

Philosophical  Magazine,  n,  p.  209,  1881. 

4Comptes  Rendus,  115,  p.  505,  1892. 

51 


52  STUDIES  IN   LUMINESCENCE. 

several  different  samples  of  which  were  tested.  Henry  states  that  in  the 
case  of  one  specimen  the  results  were  represented  by  an  exponential  law 
(eq.  i)  for  14  seconds,  while  other  preparations  obeyed  the  law 

(3) 


Henry  appears  to  have  been  convinced  of  the  correctness  of  the  latter  law 
and  proposed  a  new  type  of  photometer  which  made  use  of  the  gradually 
decaying  phosphorescence  of  Sidot  blende  in  the  measurement  of  faint 
sources  of  light.1  While  the  fact  that  the  constant  in  eq.  (3)  is  given  to 
five  significant  figures  indicates  an  accuracy  that  is  unusual  in  photometric 
measurements,  the  lack  of  experimental  data  in  the  paper  referred  to 
makes  it  difficult  to  form  an  independent  opinion  of  the  significance  of  the 
conclusions. 

The  decay  of  phosphorescence  was  considered  from  the  theoretical  stand- 
point by  H.  Becquerel2  in  1891.  Upon  the  assumption  that  the  light 
emitted  during  phosphorescence  was  due  to  molecular  vibrations  set  up 
by  the  action  of  the  exciting  light  and  afterwards  gradually  dying  out,  it 
was  shown  that  the  law  of  decay  would  be  determined  by  the  nature  of  the 
damping  forces.  If  the  vibrations  meet  with  an  opposing  force  propor- 
tioned to  the  speed  it  was  shown  that  an  exponential  law  of  decay  would 
result;  while  if  the  resistance  is  proportional  to  the  square  of  the  speed  the 
law  of  decay  would  take  the  form 


It  will  be  noticed  that  the  empirical  law  proposed  by  E.  Becquerel  reduces 
to  (4)  in  case  ^  =  0.5. 

In  the  case  of  a  substance  whose  phosphorescence  spectrum  contains 
several  bands  Becquerel  proposed  the  expression 


in  which  there  is  one  term  in  the  summation  for  each  band.  Upon  testing 
this  law  with  the  data  obtained  by  E.  Becquerel  for  a  calcium  sulphide 
giving  blue  phosphorescence  it  was  found  that  the  results  could  be  expressed 
by  the  use  of  two  terms  in  the  above  series  with  great  accuracy.  The 
existence  in  the  spectrum  of  this  substance  of  two  bands  possessing  inde- 
pendent properties  could  be  demonstrated  in  various  ways. 

In  the  derivation  of  the  law  proposed  by  H.  Becquerel  the  assumption 
is  that  the  vibrations  set  up  by  the  action  of  the  exciting  light  continue 
during  several  minutes  or  even  hours.  This  would  imply  either  that  the 
vibrating  atoms  or  molecules  exist  during  this  time  without  collisions  with 
other  molecules,  or  else  that  such  collisions  are  without  effect  upon  the 
vibrations.  Neither  of  the  suppositions  seems  to  us  tenable.  But  the 
law  nevertheless  appears  to  be  of  very  general  application.  We  shall 
show  later  that  the  same  law  may  be  derived  from  entirely  different  theo- 
retical considerations.  (See  Chapter  XV.) 

'Comptes  Rendus,  115,  p.  602. 
2Comptes  Rendus,  113,  p.  618,  1891. 


DECAY   OF   PHOSPHORESCENCE   IN    SIDOT   BLENDE.  53 

In  all  of  the  experiments  upon  the  decay  of  phosphorescence  with  which 
we  are  familiar  it  is  the  total  light  that  has  been  measured;  so  far  as  we 
are  aware  no  attempt  has  been  made  to  determine  the  law  of  decay  for 
different  portions  of  the  phosphorescence  spectrum.  This  fact  complicates 
the  problem  greatly,  for  in  most  cases  of  phosphorescence  the  spectrum 
consists  of  two  or  more  bands,  which,  in  general,  decay  at  different  rates. 
It  can  hardly  be  expected,  therefore,  that  measurements  of  the  total  light 
will  be  found  to  obey  a  simple  law.  The  difficulties  resulting  from  the 
complexity  of  the  luminescence  spectrum  were  recognized  by  E.  Becquerel 
and  were  several  times  mentioned  in  the  course  of  his  classic  researches. 
In  discussing  his  experiments  on  decay  he  suggested  an  expression  of  the 
form 

I  =  Ae~at+Be-lit+    ...  (6) 

for  the  intensity  of  the  total  phosphorescence,  with  one  term  for  each 
band  in  the  spectrum.  But  the  difficulties  of  computation  were  such  as  to 
lead  him  to  abandon  this  expression  and  to  employ  instead  the  empirical 
expression  of  eq.  (2).  Recognition  of  the  fact  that  each  band  has  its  own 
rate  of  decay  is  also  implied  in  the  law  proposed  by  H.  Becquerel  (eq.  4). 

While  it  is  possible  to  test  the  correctness  of  an  expression  of  the  form  of 
(4)  or  (6)  by  comparison  with  measurements  of  total  intensity,  such  a  test  is 
difficult,  and  can  not  be  altogether  satisfactory;  for  with  an  expression 
containing  several  terms  the  number  of  constants  is  so  great  that  the  law 
may  be  made  to  fit  almost  any  data.  It  is  clear  that  a  much  more  severe 
test  of  any  given  law  of  decay  may  be  obtained  from  experiments  with  a 
substance  having  only  one  band  in  its  phosphorescence  spectrum.  The 
Sidot  blende  screen  used  by  us  in  earlier  experiments  was  found  to  have 
three  bands  in  its  luminescence  spectrum.  But  since  the  two  violet  bands 
do  not  appreciably  overlap  the  green  band  the  spectrophotometer  enables 
the  behavior  of  the  latter  band  alone  to  be  conveniently  studied.  The 
matter  is  still  further  simplified  by  using  the  violet  rays  of  the  arc  in  excita- 
tion, since  these  rays,  while  they  produce  a  brilliant  green  luminescence,  are 
incapable  of  exciting  either  one  of  the  violet  bands.1  In  view  of  the  bril- 
liancy of  the  green  band,  its  long  duration,  and  the  ease  with  which  it  maybe 
isolated,  this  band  seems  well  adapted  to  the  study  of  the  decay  of  phos- 
phorescence. 

EARLY   STAGES   IN   THE    DECAY   OF    PHOSPHORESCENCE    IN    SIDOT    BLENDE. 

Our  previous  experiments  on  Sidot  blende  have  shown  that  in  the  case 
of  the  green  band  the  phosphorescence  spectrum  shows  no  measurable 
change  in  form  during  the  first  10  seconds  of  decay.  In  other  words,  the 
rate  of  decay  is  the  same  for  all  wave-lengths.  To  fix  the  behavior  of  the 
whole  band  it  would  be  sufficient,  therefore,  to  determine  the  law  for  a 
single  wave-length.  We  have,  however,  made  measurements  at  three 
different  wave-lengths,  namely,  at  0.483  ju,  0.512  ju,  and  0.547  M-  The  first 
of  these  wave-lengths  lies  near  the  ultra  edge  of  the  band ;  the  second  is  not 
far  from  the  maximum;  and  the  third  is  near  the  red  edge. 

'See  Chapter  III  of  this  memoir. 


54 


STUDIES   IN   LUMINESCENCE. 


The  methods  of  measurement  were  the  same  as  those  described  in  Chap- 
ter III.  The  experimental  data  obtained  are  contained  in  Tables  13,  14, 15. 
In  each  case  it  is  the  average  of  from  ten  to  twenty  observations  that 
is  recorded. 

In  the  set  of  observations  contained  in  Table  13  the  curves  for  0.547  n, 
0.483  ju,  and  0.512  n  were  determined  during  the  forenoon  in  the  order  just 
stated.  The  data  for  0.512^1  with  weaker  excitation  were  observed  during 
the  afternoon  of  the  same  day.  The  observations  recorded  in  Table  14 
were  taken  in  a  different  manner,  the  measurements  at  a  given  intensity 
being  made  for  each  of  the  three  wave-lengths  in  succession.  The  obser- 
vations of  December  13  (Table  15)  were  made  in  a  similar  manner.  The 
first  three  curves  of  Table  13  are  plotted  in  Fig.  44,  while  the  second  and 
fourth  curves  of  this  table  are  shown  in  Fig.  45. 

TABLE  13.     [Nov.  25.] 

Time  in  seconds  required  for  the  phosphorescence  to  fall  from  its  initial  intensity  to 
the  intensity  /.  The  computed  values  of  /  are  derived  by  substitution  in  the 
following  equations: 


For  X  =  o.  483/1 


X  =  o.  51  2ju  (weaker  excitation) 


i  /J  '  /  =0. 102  +  0.059* 
I /I //^= 0.074 +  0.045* 
1 1\/ 1  =0.096  +  0.055* 
I  /}•'  I  =0.092  +  0.052* 


X  =  0.  483/1 

X=o.  512/1 

X  =  0.547M 

X  =  O.SI2M 
Weaker  excitation. 

/ 

/sec. 

t  sec. 

/  sec. 

/sec. 

/  sec.         t  sec. 

/sec. 

/  sec. 

Obs. 

Comp. 

Obs. 

Comp. 

Obs.        Comp. 

Obs. 

Comp. 

101 

o  63 

o  58 

' 

81 

o  8} 

o  85 

61 

0.41 

0.44 

1.38 

I  .20 

0.56    !       0.58 

0.72 

0.69 

4' 

0.99 

0.92 

1.84 

1.83 

i  .  10       i  .09 

I  .24         I  .23 

21 

1.88 

..98 

3-25 

3-23 

2.07          2.23 

2.42   i     2.45 

11 

3.16 

3-39 

4-95 

5.  10 

3.87          3.74 

4.03   j     4.04 

6 

6-55 

5-23 

7.42 

7.50 

5.82           5.71 

6.22         6.92 

TABLE  14.     [Dec.  10.] 

Time  in  seconds  required  for  the  phosphorescence  to  fall  to  the  intensity  7.     The 
computed  values  of  t  are  derived  by  substitution  in  the  following  equations: 

For  X  =  o.5i2/u     i /\// =0.078+0.034*     X  =  o.  547^1     i /-y/7  =o.  104+0. 042^ 


I 

X=o.  483/1 

X=0.  512/1 

X=0.547M 

/sec. 
Observed. 

/sec. 
Computed. 

/  sec. 
Observed. 

/  sec. 
Computed. 

/sec. 
Observed. 

/sec. 
Computed. 

81.4 

61  .4 
3i-4 
...4 

O.6o 

0-77 
2.40 

4.0 

1.06 

1-35 

2.98 
6.47 

0.98 
1.49 
3.O2 
6.46 

0.30 
0.55 
1.83 
4.62 

O.22 
0.62 
..84 
4.58 

DECAY   OF   PHOSPHORESCENCE   IN   SIDOT   BLENDE. 


55 


TABLE  15.    [Dec.  13.] 

Time  in  seconds  required  for  the  phosphorescence  to  fall  to  the  intensity  /.    The 
computed  values  of  t  are  derived  by  substitution  in  the  following  equations: 

i  /V/_=o.  104+0.070* 
i /V/^=  0.094+0. 046* 
i  /V/  =o.  1 18+0.070* 


/ 

X  =  o.483M 

X=o.  512/1 

X=o.  547/1 

/sec. 
Observed. 

/sec. 
Computed. 

/sec. 
Observed. 

/sec. 
Computed. 

/sec. 
Observed. 

/see. 
Computed. 

81.4 

61  .4 

3'-4 

21.4 
11.4 
7-4 

0.45 
0.69 

0.37 
0.74 

0.45 
I  .06 
I  .46 

2.78 
4.46 

0.36 
1.  08 
1.  61 
2.76 
3.80 

0.25 
0.88 
1.30 

2.61 

4.58 

o.  14 

0.87 
1-40 
2-55 

3-57 

2.6o 
4.48 
5-97 

2.67 

4-44 
6.00 

The  data  of  the  preceding  tables  have  been  used  to  test  the  applicability 
of  each  of  the  several  proposed  laws  of  decay  to  the  case  of  a  single  band. 
It  was  found  that  the  results  can  be  closely  represented  by  an  expression 
of  the  form  given  in  eq.  (4).  To  determine  the  constants  a  and  b  of  this 
equation  a  convenient  graphical  method  was  employed,  in  which  the  values 
of  //  V7  were  plotted  as  ordinates  and  times  as  abcissae.  Since  eq.  (4)  may 
be  written  in  the  form 


the  points  located  in  this  manner  should  lie  on  a  straight  line.  A  straight 
line  having  been  drawn  as  nearly  as  possible  through  all  the  points,  the 
slant  of  this  line  and  its  y  intercept  at  once  gave  the  values  of  b  and  a  respec- 
tively. The  values  of  a  and  b  determined  in  this  way  are  given  in  each  of 
the  tables,  and  the  values  of  /  computed  from  the  above  equation  are 
tabulated  for  comparison  with  the  times  observed. 


Fig.  44. 

Curves  .4,  B,  and  C  show  the  decay  in 
the  phosphorescence  of  Sidot  blende 
for  the  wave-length  0.483/1.  0.512  /», 
and  0.547  p,  respectively.  The  curves 
A',  B',  and  C'  are  obtained  from  A, 
B.  and  C  by  plotting  /—'a  instead  of 
the  intensity  of  phosphorescence,  /. 


345 
Seconds 


In  Figs.  44  and  45  the  values  of  7  *  have  been  plotted  as  described 
above.     It  will  be  noticed  that  in  each  case  the  points  lie  very  nearly  upon 


STUDIES  IN  LUMINESCENCE. 


a  straight  line,  and  that  there  is  nothing  to  indicate  any  systematic  vari- 
ation from  linearity.  In  the  case  of  one  curve,  namely,  that  for  0.483  M 
in  Table  14,  the  points  located  in  this  way  did  not  lie  even  approximately 
on  a  straight  line.  This  curve,  therefore,  does  not  have  the  form  corre- 
sponding to  eq.  (4).  But  in  the  case  of  the  other  curves,  including  one 
taken  at  this  same  wave-length  (Table  13),  the  agreement  between  the 
observed  and  computed  values  of  t  is  satisfactory,  the  differences  being 
only  such  as  might  well  result  from  accidental  errors.  It  seems  probable, 
therefore,  that  the  observations  at  0.483  /j.  in  Table  14  were  subject  to  some 
unusual  source  of  error. 

In  Fig.  44  it  will  be  noticed  that  one  point  of  the  line  A '  (indicated  by  a 
question  mark)  falls  below  the  line,  i.  e.,  the  observed  time  of  decay  was 
considerably  greater  than  that  computed  from  an  assumed  linear  relation 
between  7~*  and  t.  Discrepancies  of  the  same  kind  will  be  found  for  the 
smallest  values  of  /  in  the  first  curve  of  Table  13  and  for  the  first  and  last 


Fig-  45- 

Curves  .A  and  B  show  the  decay  of  phos- 
phorescence at  0.512/1  for  two  dif- 
ferent intensities  of  the  exciting  light. 
In  the  curves  A'  and  B'  1~%  is  plotted 
instead  of  the  intensity  /. 


Seconds 


curves  of  Table  15.  No  such  discrepancies  are  present  in  any  of  the  four 
curves  taken  at  0.512  n  —  i.  e.,  near  the  maximum  of  the  phosphorescence 
spectrum.  If  these  cases  of  disagreement  between  the  observed  and  com- 
puted values  of  I  are  not  due  to  accidental  errors,  they  show  the  beginning 
of  a  systematic  variation  which  will  be  dealt  with  at  length  in  a  later 
paragraph. 

£  The  theory  discussed  in  Chapter  XV  leads  us  to  expect  that  under  ideally 
simple  conditions  the  decay  of  phosphorescence  should  be  in  accordance 
with  the  equation 


where 


-j= 

v/ 


a  = 


a 
fr-vh: 


It  is  to  be  remembered  that  if  I  refers  to  the  intensity  at  some  particular 
part  of  the  phosphorescence  spectrum,  k  must  be  regarded  as  a  function 
of  the  wave-length. 

The  fact  that  the  experiments  indicate  a  linear  relation  between  the  time 
and  7~*  has  already  been  pointed  out,  and  reference  to  the  tables  on  pages 


DECAY   OF   PHOSPHORESCENCE    IN    SIDOT   BLENDE.  57 

54  and  55  shows  that  the  differences  between  the  computed  and  observed 
values  of  /,  which  are  usually  small,  are  entirely  unsystematic.  It  is 
possible  to  test  the  law  still  further  by  considering  the  values  of  the  con- 
stants a  and  b.  Except  in  the  case  of  the  last  curve  of  Table  13,  the  data 
refer  to  curves  of  decay  for  different  wave-lengths,  but  with  the  same 
excitation.  a  and  «0  are  therefore  constant,  while  k  depends  upon  the 
wave-length.  It  will  be  noticed  that 

a  i  \k         i 


b       woVjfca'a       n<>a 

This  quotient  should  therefore  be  a  constant  for  all  curves  taken  with  the 
same  excitation  and  under  similar  experimental  conditions.  The  values 
of  the  ratio  a/b  for  the  three  curves  of  Table  15  show  considerable  vari- 
ation from  equality,  being  1.5,  2.0,  and  1.7.  The  values  of  the  ratio  for  the 
first  three  curves  of  Table  13  are  1.73,  1.75,  and  1.65.  In  view  of  the 
difficulty  of  maintaining  constant  conditions  in  the  experiments  on  phos- 
phorescence the  agreement  between  these  three  values  is  highly  satisfactory. 
As  already  stated,  the  observations  at  0.483  ;u  in  Table  14,  probably  on 
account  of  some  experimental  error,  can  not  be  represented  by  the  expres- 
sion here  considered,  so  that  the  value  of  a/b  for  this  curve  can  not  be  deter- 
mined. But  for  the  curves  taken  at  0.512/1  and  0.547/1  (Table  14)  the 
quotient  a/b  has  the  values  2.32  and  2.43,  respectively.  Here,  too,  the 
agreement  is  all  that  could  be  expected.1  Our  results  therefore  afford 
strong  confirmation  not  only  of  the  conclusion  that  all  parts  of  the  green 
band  decay  at  the  same  rate,  but  also  of  the  general  theory  of  phosphor- 
escence that  we  have  used  in  deriving  the  law  of  decay. 

The  data  permit  still  another  test  of  the  general  theory,  the  result  of 
which  is  less  satisfactory.  It  will  be  remembered  that  in  the  case  of  the 
fourth  curve  of  Table  13  the  exciting  light  was  less  intense  than  that  used 
with  the  other  curve  for  0.512  /*.  The  observations  for  the  fourth  curve 
were  made  on  the  same  day  as  those  for  the  second  curve,  and  although  the 
former  were  made  in  the  afternoon  and  the  latter  in  the  forenoon,  every 
attempt  was  made  to  keep  the  comparison  source  the  same  in  the  two  cases 
and  to  have  all  the  other  experimental  conditions  as  nearly  as  possible 
constant.  If  we  compare  the  two  curves  for  0.512/1  in  Table  13,  we  see 
that  a  and  k  have  the  same  value  for  both,  while  n0  is  different.  It  would 
seem  reasonable,  however,  to  regard  the  constant  b  as  independent  of  «<>• 
This  constant  should  therefore  have  the  same  value  for  both  curves ;  in  other 
words,  the  straight  lines  obtained  by  plotting  7-i  should  be  parallel.  As  a 
matter  of  fact  the  values  of  b  determined  for  the  two  curves  are  0.045,  f°r 
the  second  curve  of  Table  13,  and  0.052  for  the  fourth.  The  fact  that  the 
two  values  are  unequal  is  shown  in  Fig.  45  by  the  lack  of  parallelism  of  the 
two  lines  A'  and  B'. 

Both  the  theory  of  Becquerel  and  the  simple  theory  first  discussed  in 
Chapter  XV  fail  to  explain  this  result;  which, however, is  in  accord  with  the 
modified  theory  of  Wiedemann  and  Schmidt  discussed  in  the  latter  part  of 
Chapter  XV. 


'There  is  no  reason  why  the  value  a/b  for  Table  13  should  be  the  same  as  those  for  Table  14  or  15,  since 
the  observations  were  made  several  weeks  apart,  and  with  no  attempt  to  keep  the  intensity  of  the  com- 
parison source  the  same  in  the  different  cases. 


58  STUDIES   IN   LUMINESCENCE. 

THE  STUDY  OF  PHOSPHORESCENCE  IN  SIDOT  BLENDE  AND  CERTAIN  OTHER 
SUBSTANCES  DURING  THE  WHOLE  PERIOD  OF  DECAY. 

In  the  experiments  just  described  the  phosphorescence  of  Sidot  blende 
was  followed  by  the  spectrophotometer  for  about  10  seconds  after  the 
cessation  of  excitation.  At  the  end  of  this  period  the  intensity  of  phos- 
phorescence had  fallen  to  about  4  per  cent  of  its  original  value.  Although 
it  was  possible  to  see  the  phosphorescent  light  in  the  spectrophotometer 
for  a  much  longer  period  than  this,  the  extreme  faintness  of  the  field  made 
accurate  measurements  out  of  the  question.  In  attempting  to  determine 
the  law  of  decay  throughout  a  wider  range  we  have  therefore  been  com- 
pelled to  abandon  the  use  of  the  spectrophotometer  and  to  measure  the 
total  light. 

Measurements  of  the  total  intensity  do  not,  in  general,  afford  a  satis- 
factory means  of  determining  the  law  of  decay  of  phosphorescence,  since,  as 
has  already  been  pointed  out,  the  phosphorescence  spectrum  usually  con- 
tains two  or  more  bands,  which  decay  at  different  rates.  We  have  already 
referred  to  the  difficulties  that  are  met  with  in  interpreting  the  results  of 
such  measurements.  In  the  case  of  Sidot  blende  the  conditions  are,  how- 
ever, relatively  simple,  for  the  phosphorescence  of  the  green  band  is  so  much 


Fig.  46. 

more  prominent,  both  in  duration  and  in  intensity,  than  that  of  the  violet 
bands  that  the  presence  of  these  does  not  appreciably  affect  the  total 
intensity.  Measurements  of  the  total  phosphorescence  of  Sidot  blende  are 
therefore  practically  measurements  of  the  intensity  of  the  green  band  only. 
If  all  parts  of  this  band  decay  at  the  same  rate,  measurements  of  total  in- 
tensity will  give  the  same  results  as  measurements  taken  at  some  particular 
portion  of  the  band.  Our  earlier  observations  have  shown  that  all  parts  of 
the  green  band  of  Sidot  blende  do  decay  at  the  same  rate  for  at  least  10  seconds. 
It  seems  not  unlikely,  therefore,  that  this  equality  of  rates  will  be  maintained. 
Although  the  present  section  deals  chiefly  with  the  phosphorescence  of 
Sidot  blende,  the  results  obtained  in  the  study  of  several  other  substances 
are  also  included. 

EXPERIMENTAL,  METHOD. 

The  apparatus  used  is  shown  in  Fig.  46.  The  white  screen  was  removed 
from  a  Lummer-Brodhun  photometer  and  the  Sidot  blende  screen,  5,  was 
put  in  its  place,  the  active  surface  of  the  screen  being  toward  the  left.  The 
screen  was  usually  excited  by  a  Lummer  mercury  lamp,  L,  of  the  "end  on " 
form.  In  some  instances  the  carbon  arc,  and  in  other  cases  the  spectrum 
of  the  carbon  arc,  was  used  for  excitation.  The  exciting  light  could  be  cut 
off  by  means  of  a  shutter,  S,  which  at  the  same  time  made  a  record  on  a 


DECAY   OF   PHOSPHORESCENCE    IN    SIDOT   BLENDE-  59 

chronograph.  When  the  mercury  lamp  was  used  in  excitation  a  piece  of 
blue  glass,  B,  was  placed  between  the  lamp  and  the  screen,  so  that  only  the 
violet  lines  in  the  spectrum  were  used. 

The  right-hand  side  of  the  screen  was  of  white  paper  and  could  be 
illuminated  by  means  of  the  acetylene  lamp  A.  Two  pieces  of  green 
glass,  G,  served  to  produce  a  sufficient  color  match.  The  acetylene  flame 
A  was  in  a  metal  box,  and  the  light  used  emerged  from  a  small  opening 
immediately  in  front  of  the  central  part  of  the  flame.  Small  changes  in 
pressure  were  therefore  without  effect  upon  the  intensity  of  this  comparison 
source. 

The  procedure  in  taking  observations  was  as  follows: 
One  observer,  with  his  eyes  suitably  protected  from  stray  light,  watched 
the  decay  of  the  phosphorescence  after  the  shutter  had  been  closed,  and 
recorded  by  means  of  a  key  the  instant  at  which  the  phosphorescent  light 
and  the  comparison  field  were  equal.  The  second  observer  then  shifted 
the  position  of  the  comparison  flame  A  to  a  slightly  greater  distance  from 
the  photometer,  and  the  time  when  equality  was  again  reached  was  recorded 
as  before.  In  this  way  it  was  often  possible  to  get  as  many  as  15  points 
on  the  curve  of  decay,  although  a  smaller  number  than  this  was  more 
usual. 

EXPERIMENTS   WITH   SIDOT  BLENDE. 

In  preliminary  experiments  the  light  from  the  carbon  arc  was  used  in 
excitation.  It  was  thought  that  simpler  conditions  would  be  obtained 
if  exciting  light  having  only  a  small  range  of  wave-lengths  was  used,  and  a 
spectrum  of  the  arc  was  therefore  formed,  the  violet  end  of  which  was  used 
for  excitation.  The  gain  in  the  intensity  of  phosphorescence  which  re- 
sulted from  cutting  off  the  red  and  infra-red  rays  was  very  noticeable. 

In  our  previous  experiments  with  Sidot  blende  it  was  found  that  the 
decay  of  phosphorescence  during  the  first  7  seconds  was  in  accordance 
with  the  law 

J= 


so  that  upon  plotting  the  curves  with  T  and  /~-  as  coordinates  a  straight 
line  was  obtained.  Since  the  distance  between  the  photometer  screen 
and  the  acetylene  flame  is  proportional  to  /"*,  it  was  convenient  to  plot 
the  results  of  the  present  experiments  in  the  same  way. 

The  general  character  of  the  decay  curves  when  plotted  in  this  manner 
is  exhibited  by  the  curves  of  Fig.  47.  The  violet  end  of  the  carbon  arc 
spectrum  was  used  in  excitation,  and  the  duration  of  excitation  was  varied 
as  indicated. 

Inspection  of  Fig.  47  shows  that  while  each  curve  starts  as  a  line  which 
is  nearly  straight  in  the  neighborhood  of  the  origin,  it  soon  begins  to  bend 
over  and  ultimately  changes  into  a  second  straight  line  having  a  different 
slant.  This  behavior  is  especially  noticeable  in  the  case  of  long  exposures. 

If  phosphorescence  is  due  to  the  recombination  of  ions  that  have  been 
separated  by  the  action  of  the  exciting  light,  as  assumed  in  the  theory  of 
Wiedemann  and  Schmidt,  it  appears  that  the  coefficient  of  recombination 
does  not  remain  constant  during  the  whole  period  of  decay.  During  the 


6o 


STUDIES  IN   LUMINESCENCE. 


first  6  or  7  seconds  this  coefficient  is  practically  constant,  as  was  shown  by 
the  results  of  our  previous  experiments.  A  change  then  begins  which  con- 
tinues for  about  20  or  30  seconds,  after  which  the  curves  indicate  a  new 
coefficient,  smaller  than  before.  This  new  coefficient,  as  is  shown  by  the 
constant  slant  of  the  curves  of  Fig.  47,  remains  constant  during  the  rest  of 
the  period  of  decay,  or  at  least  until  the  phosphorescence  has  become  too 
faint  to  measure.  Whatever  theoretical  interpretation  of  the  curves  is 


Fig-  47- 


Effect  of  duration  of  excitation. 
Violet  end  of  carbon-arc  spec- 
trum used  in  excitation.  The 
curves  were  taken  in  the 
order  indicated  by  the  letters. 
The  times  of  excitation  were 
as  follows:  Curve  A,  0.9.; 
B.  2.4  sec.;  C,  4.5  sec.;  D,  9.2 
sec.;  £,  17.5  sec. 


adopted  it  is  clear  that  the  decay  of  phosphorescence  involves  two  distinct 
processes  which  merge  into  one  another.  In  each  of  these  the  decay  obeys 
the  simple  law  already  referred  to,  but  the  rate  at  which  the  phosphorescence 
dies  out  is  greater  in  the  first  process  than  in  the  second. 

It  will  be  observed  that  the  slant  of  the  curve,  for  each  of  the  two  pro- 
cesses, is  a  function  of  the  duration  of  excitation. 


Fig.  48. 

Effect  of  duration 
of  excitation. 
Violet  light  of 
mercury  arc 
used  for  exci- 
tation. The 
curves  were 
taken  in  the 
order  by  the 
letter.  The 
times  of  excita- 
tion  were  as 
follows :  Curve 
A,  27  sec.;  B, 
10  sec.;  C,  3.1 
sec.;  D,  1.2  sec. 


Seconds 

In  Fig.  48  is  shown  a  series  of  curves  taken  under  the  same  conditions  as 
those  of  Fig.  47,  except  that  the  mercury  arc  was  used  in  excitation  instead 
of  the  violet  end  of  the  ordinary  arc  spectrum.  One  is  immediately  struck 
by  the  difference  between  the  two  sets  of  curves.  In  Fig.  48  there  is  a 
strong  tendency  toward  parallelism  in  the  parts  of  curves  corresponding 
to  the  second  process.  In  Fig.  47  no  such  tendency  is  observable. 


DECAY   OF   PHOSPHORESCENCE    IN   SIDOT   BLENDE. 


6l 


We  were  at  first  inclined  to  ascribe  this  difference  to  the  fact  that  different 
kinds  of  exciting  rays  had  been  used  in  the  two  cases ;  but  the  two  sets  of 
curves  differ  in  another  respect,  namely,  in  the  order  in  which  the  curves 
were  taken.  In  the  first  case  the  curves  of  short  excitation  were  taken 
first,  while  in  the  latter  case  the  curves  corresponding  to  long  excitations 
were  the  first  observed.  It  appeared  possible  that  the  difference  in  the 
form  of  the  curves  was  due  to  this  difference  in  sequence  rather  than  to  the 
difference  in  exciting  light. 

To  test  this  matter  the  observations  plotted  in  Fig.  49  were  made.  It 
will  be  noticed  that  the  dotted  curves  are  similar  to  those  in  Fig.  48,  while 
the  full  curves  are  similar  to  those  of  Fig.  47.  The  mercury  arc  was  used 
in  excitation  in  all  cases.  But  curves  taken  after  the  screen  had  been  sub- 
jected to  the  long  excitations  corresponding  to  curves  C  and  D  differ  widely 
from  the  curves  taken  with  approximately  the  same  excitation  previous 
to  C  and  D.  A  comparison  of  curves  A  and  E  illustrates  this  point,  the 
the  duration  of  exposure  being  exactly  the  same  in  each  of  these  two  cases. 


Fig.  49. 

Effect  of  duration  of 
excitation.  Mercury 
arc.  The  curves  were 
taken  in  the  order  in- 
dicated by  the  letters. 
The  times  of  excita- 
tion were  as  follows: 
Curve  A ,  4.3  sec. ;  B, 
8.2  sec.;  C,  16.0  sec.; 
D.  56.0  sec.;  E,  4.3 
sec.;  F,  1.4  sec.;  G, 
i  .o  sec. 


These  results  indicate  that  some  change  is  produced  in  the  phosphorescent 
material  by  the  action  of  the  exciting  light,  and  that  this  changed  condition 
persists  for  a  considerable  period  after  all  visible  phosphorescence  has 
ceased.  In  other  words,  the  effect  of  a  given  excitation  in  producing  phos- 
phorescence depends  upon  the  previous  history  of  the  phosphorescent 
substance. 

If  the  screen  is  allowed  to  rest  in  the  dark  for  a  number  of  hours  this 
semi-permanent  effect  of  exposure  in  part  dies  out.  But  rest  alone  does 
not  restore  the  screen  completely,  even  if  continued  for  several  days.  The 
effect  of  rest  was  also  found  to  be  somewhat  uncertain,  being  much  greater 
on  some  occasions  than  on  others. 

Several  methods  of  restoring  the  screen  to  a  standard  condition  were 
tried.  Heating  the  screen  to  the  temperature  of  boiling  water  for  several 
minutes  and  then  cooling  it  again  to  the  temperature  of  the  room  seemed 
effective.  But  this  method  required  considerable  time  and  has  not  been 
thoroughly  tested.  Cooling  the  screen  to  the  temperature  of  liquid  air  and 
afterwards  warming  it  gradually  to  the  original  temperature  seemed  to  be 
without  effect. 


62 


STUDIES   IN   LUMINESCENCE. 


The  well-known  effect  of  the  red  and  infra-red  rays  in  suppressing  the 
long-time  phosphorescence  of  various  substances  led  us  to  think  that  these 
rays  might  also  prove  effective  as  a  means  of  restoring  the  screen  to  a 
standard  condition.  This  conjecture  proved  to  be  correct,  and  exposure 
to  the  red  and  infra-red  rays  of  a  50  c.p.  lamp  at  a  distance  of  about  20  cm. 
was  found  to  be  both  convenient  and  satisfactory.  A  piece  of  ruby  glass 
placed  in  front  of  the  lamp  served  to  remove  the  more  refrangible  rays. 
An  exposure  of  a  few  seconds  to  the  rays  that  passed  through  the  ruby  glass 
was  sufficient  to  bring  the  screen  into  what  seemed  to  be  a  definite  standard 
condition.  A  longer  exposure  was,  however,  ordinarily  used. 

The  changes  produced  by  excitation  and  the  effect  of  the  red  and  infra- 
red rays  are  illustrated  by  the  curves  shown  in  Fig.  50.  Curve  A  shows 
the  behavior  of  the  screen  when  exposed  for  10  seconds  after  resting  in  the 
dark  for  24  hours.  Curve  B  is  that  corresponding  to  an  exposure  of  2 
minutes;  curve  C  was  taken  with  an  exposure  of  10  seconds  immediately 


no  S«c, 


Fig.  50. — Illustrating 
relative  effect  of  rest 
and  exposure  to  infra 
red. 

A,  10  sec.  excitation  after 
rest  of  24  hours  in  the 
dark.  B,  2  min.  excita- 
tion. 

C,  10   sec.    excitation   im- 
mediately after  B. 

D,  10  sec.    excitation  after 
exposure  of  4  min.  to  infra- 
red from  50  candle-power 
lamp. 

Curves  A'.  B',  C',  D'  cor- 
respond to  A,  B,  C.  D. 
except  that  /  is  plotted 
in  place  of  /~-*». 


after  curve  B;  and  curve  D,  also  with  an  exposure  of  10  seconds,  was  taken 
after  the  screen  had  been  exposed  to  the  red  and  infra-red  rays  for  4  minutes.1 
In  some  respects  the  behavior  of  the  screen  is  analogous  to  the  magnetic 
behavior  of  iron.  When  iron  is  magnetized  a  certain  residual  magnetization 
remains  after  the  removal  of  the  magnetizing  force,  and  the  effect  produced 
by  a  subsequent  magnetizing  force  depends  upon  the  magnetic  history  of 
the  specimen.  Similarly  some  change  is  produced  in  Sidot  blende  by  excita- 
tion which  does  not  immediately  disappear  upon  the  removal  of  the  excit- 
ing light,  and  which  modifies  the  effect  produced  by  subsequent  excitation. 
The  analogy  is  rendered  more  striking  if  this  property  of  Sidot  blende  is 
exhibited  in  a  different  manner,  as  in  Fig.  51.  In  the  curve  plotted  in  this 
figure  the  abscissa  of  each  point  shows  the  duration  of  excitation,  while 
the  ordinate  gives  the  corresponding  intensity  of  phosphorescence  after  30 
seconds  decay.  The  resemblance  of  the  curve  to  a  hysteresis  loop  for  iron 
is  striking. 


Experiments  dealing  with  the  effect  of  the  infra-red  rays  during  and  after  excitation  will  be  found  in 
Chapter  V  of  this  memoir. 


DECAY   OF   PHOSPHORESCENCE    IN   SIDOT   BLENDE. 


It  seems  possible  that  the  action  of  these  rays  in  destroying  the  residual 
effect  in  the  phosphorescent  substance  is  similar  to  the  effect  of  jarring  or 
tapping  in  destroying  the  residual  magnetism  of  a  bar  of  iron.  Ignorance 
of  the  existence  of  hysteresis  would  evidently  lead  to  confusing  results  in 
the  case  of  either  of  these  two  classes  of  phenomena.  Our  delay  in  recog- 
nizing the  effect  of  previous  history  has  in  fact  made  it  necessary  for  us  to 
discard  all  of  our  earlier  observations. 


Fig-  5i- 

Hysteresis  loop.  Ordinates 
give  the  intensity  of  phos- 
phorescence 30  seconds 
after  the  end  of  excitation. 
The  curves  from  which  these 
points  were  determined 
were  observed  in  the  order 
indicated  by  the  arrows. 


40  60  80 

Duration  of  excitation 


100  sec 


In  our  later  work  the  screen  was  exposed  to  the  red  and  infra-red  rays 
as  described  for  one  minute  before  each  exposure.  With  this  precaution 
to  avoid  the  effects  of  hysteresis  the  curves  shown  in  Fig.  52  were  taken 
to  determine  the  effect  of  varying  times  of  exposure. 


o? 


ZO  40  6O  80  IOO  IZO  I4-O  I«O 

Seconds 
Fig-  52. 

Effect  of  duration  of  excitation.  Violet  of  mercury  arc  used  for  excitation.  Screen  exposed  to 
infra-red  for  i  minute  before  each  curve.  The  times  of  excitation  were  as  follows: 
Curve  A,  1.2  sec.;  B,  5.4  sec.;  C,  12.0  sec.;  D,  37  sec.;  E,  60  sec.;  F,  15  min. 

Since  our  previous  experiments  have  shown  that  the  curves  are  accurately 
straight  in  the  neighborhood  of  t  =  o,  it  is  possible  to  determine  the  initial 
intensity  of  phosphorescence  by  prolonging  the  curves  in  each  case  until 


64 


STUDIES  IN  LUMINESCENCE. 


they  strike  the  vertical  axis.  From  the  intercept  thus  determined,  which 
is  equal  to  /o~*,  the  initial  intensity  can  be  computed.  From  the  results 
obtained  in  this  way  from  the  data  shown  in  Fig.  52,  curve  A  in  Figs.  53 
and  54  has  been  plotted.  In  these,  two  figures  /  has  been  plotted  instead 
of  /-*. 

It  will  be  noticed  that  the  intensity  of  phosphorescence  at  first  increases 
rapidly  with  increased  duration  of  exposure,  but  that  after  an  exposure 
of  2  or  3  minutes  is  reached  there  is  little  further  change.  The  phospho- 


V 


(5        Minu+»« 


Fig-  53- 


Effect  of  duration  of  excitation.  Curve  A  shows  initial  intensity  of  phosphorescence  (/)  as  a  function 
of  the  time  of  excitation.  Curves  a,b,  and  c  show  the  decay  of  phosphorescence  after  excitations  of  15 
minutes,  5  minutes,  and  i  minute,  respectively. 

rescence  may  be  said  to  be  saturated  so  far  as  the  effect  of  duration  of 
excitation  is  concerned.  Not  only  is  the  initial  intensity  unaltered  by 
longer  excitation,  but  the  form  of  the  decay  curve  also  remains  constant, 
as  is  indicated  by  curves  a  and  b  in  Fig.  53.  (These  curves  correspond 
to  curves  F  and  E  of  Fig.  51.) 

We  have  also  studied  to  some  extent  the  influence  of  the  intensity  of 
the  exciting  light  upon  the  form  of  the  decay  curves.     In  order  to  vary 


Fig-  54- 

Effect  of  duration  of  excitation.     A  portion  of  the  same  curve  shown  in  Fig.  54  plotted  to  a  larger  scale. 

the  intensity  of  the  exciting  light  several  metal  stops  were  prepared,  which 
could  be  placed  immediately  in  front  of  the  mercury  lamp.  The  apertures 
of  these  varied  from  i  mm.  in  diameter  to  the  full  size  of  the  mercury -lamp 
tube,  namely,  15  mm.  To  determine  the  intensity  of  the  exciting  light 
corresponding  to  each  of  these  the  following  photometric  method  was 
used:  The  phosphorescent  screen  was  removed  from  the  photometer  and 
a  piece  of  white  cardboard  was  put  in  its  place.  This  being  illuminated 


DECAY   OF   PHOSPHORESCENCE    IN   SIDOT   BLENDE. 


by  the  violet  light  from  the  mercury  arc  passing  through  the  stop  whose 
constant  was  to  be  determined,  the  intensity  was  measured  by  shifting 
the  position  of  the  comparison  flame  on  the  opposite  side  of  the  photometer. 
vSuitable  glass  screens  were  used  to  equalize  the  colors  on  the  two  sides. 
To  avoid  errors  resulting  from  the  flickering  of  the  mercury  arc  ten  settings 
were  made  for  each  determination. 


Fig-  55- 

Effect  of  varying  the  intensity  of  the 
exciting  light.  Exposure  in  each  case 
20  seconds.  The  relative  intensity  of 
the  exciting  light  is  marked  on  each 
curve. 


60  80 

Seconds 


In  Fig.  55  decay  curves  are  shown  for  different  intensities  of  the  exciting 
light,  the  excitation  in  each  case  lasting  for  20  seconds.  In  Figs.  56  and  57 
similar  sets  of  curves  are  shown  for  which  the  excitations  were  respectively 
40  seconds  and  2  minutes. 

A  study  of  these  curves  shows  that  there  is  some  approach  to  intensity 
saturation;  in  other  words,  the  intensity  of  phosphorescence  is  nearly 


400 


200 


Fig.  56- 

Effect  of  varying  the  intensity 
of  the  exciting  light  (as  indi- 
cated on  each  curve).  Ex- 
posure in  each  case  40  seconds. 


proportional  to  the  intensity  of  the  exciting  light  for  small  values  of  the 
latter,  but  increases  less  rapidly  than  the  excitation  when  the  exciting 
light  is  strong.  This  point  is  well  brought  out  by  curve  A  in  Fig.  58,  in 
which  the  ordinates  are  proportional  to  the  initial  intensity  of  phosphores- 
cence, /o.  The  values  of  70  were  determined  from  the  data  of  Figs.  55,  56, 
and  57  by  extrapolation,  upon  the  assumption  that  the  relation  between 


66 


STUDIES   IN  LUMINESCENCE- 


/  and  I  -  is  linear  for  small  values  of  /.  Since  the  early  portion  of  each 
decay  curve  is  chiefly  determined  by  the  first  two  or  three  points,  which 
are  the  most  difficult  to  observe,  the  values  plotted  for  /o  are  subject  to 
considerable  error.  Curve  A  is  nevertheless  reasonably  smooth  and 
indicates  nearly  exact  proportionality  between  intensity  of  excitation  and 


100 


80  100 

Seconds 
Fig.  57- 

Effect  of  varying  the  intensity  of  the  exciting  light  (as  indicated  on  each  curve). 
Exposure  2  minutes  in  each  case. 

initial  phosphorescence.     It  is  only  with  the  most  intense  excitation  used 
that  saturation  begins. 

It  is  to  be  observed  that  in  most  cases  the  values  of  70  that  are  computed 
from  the  data  of  Fig.  55  lie  on  the  same  curve  as  those  obtained  from 
the  data  of  Fig.  57.  A  well-marked  difference  is  noticeable  only  in 


300 
I 

£00 


Fig:58. 

Effect  of  the  intensity  of  the  excitation  upon 
the  initial  intensity  of  phosphorescence.  The 
points  marked  by  dots  are  for  an  exposure  of 
2  minutes;  those  indicated  by  crosses  are  for 
an  exposure  of  20  seconds.  In  curves  B  and 
C  the  ordinates  show  the  intensity  of  phos- 
phorescence i  minute  after  excitation  had 
ceased. 


40 


Intensity  of  excitation 

the  case  of  the  points  corresponding  to  intense  excitation.  In  other  words, 
for  weak  excitations  the  intensity  of  the  initial  phosphorescence  is  the  same 
after  an  exposure  of  20  seconds  as  for  i  or  2  minutes.  There  at  first  appears 
to  be  a  contradiction  here  to  the  results  shown  in  Figs.  53  and  54.  But 
while  this  may  be  due  to  the  uncertainty  in  the  values  of  /0,  it  is  readily 
explained  upon  the  assumption  that  a  weak  excitation  produces  its  full 


DECAY   OF   PHOSPHORESCENCE   IN   SIDOT   BLENDE.  6j 

effect  more  promptly.  The  form  of  the  curves  shown  in  Figs.  53  and  54 
is  probably  largely  dependent  upon  the  intensity  of  the  exciting  light. 

The  ordinates  of  curves  B  and  C  (Fig.  58)  show  the  intensity  of  phos- 
phorescence i  minute  after  the  exciting  light  was  cut  off.  For  curve  B  the 
exposure  was  2  minutes,  while  for  curve  C  the  exposure  was  20  seconds. 
The  effect  of  duration  of  exposure  is  here  well  marked. 

In  the  case  of  exposures  lasting  for  several  seconds  or  more  the  phenomena 
are  manifestly  complicated  by  the  fact  that  the  semi-permanent  change, 
to  which  we  have  already  referred,  is  taking  place  in  the  active  substance 
during  the  time  of  excitation.  It  seemed  probable  that  the  relation 
between  intensity  of  excitation  and  intensity  of  phosphorescence  might 
prove  simpler  if  the  duration  of  excitation  was  reduced  to  a  minimum.  A 
series  of  curves  was  therefore  taken  with  a  spark  as  exciting  source.  Pre- 
liminary trials  showed  that  the  most  intense  excitation  was  furnished  by 
discharging  eight  large  jars  through  a  spark  gap  about  2  cm.  long  with  one 
cadmium  terminal.  The  distance  of  the  spark  gap  from  the  screen  was 
varied  from  about  10  cm.  to  35  cm.  A  single  spark  at  10  cm.  distance  gave 
an  excitation  approximately  equivalent  to  30  seconds  exposure  to  the  mer- 
cury arc. 

Experiments  with  the  practically  instantaneous  excitation  produced  by 
a  single  spark  showed  that  the  phosphorescence  was  proportional  to  the 
intensity  of  excitation,  not  merely  initially  but  throughout  the  whole 
period  of  decay.  In  other  words,  it  was  possible  to  bring  a  decay  curve 
determined  with  the  spark  at  a  distance  d^  into  coincidence  with  the  curve 
corresponding  to  the  distance  d\  by  multiplying  each  ordinate  72  by  the 
ratio  dz2/di2. 

EXPERIMENTS   WITH   DIFFERENT    PHOSPHORESCENT   SUBSTANCES. 

It  is  natural  to  inquire  whether  the  complex  phenomena  exhibited  by 
Sidot  blende  are  peculiar  to  this  particular  material,  or  whether  its  behavior 
is  typical  of  a  large  class  of  phosphorescent  substances.  In  order  to  test 
this  matter  we  have  determined  the  decay  curve  under  similar  conditions 
with  three  other  substances,  namely,  "  Emanations-pulver, "!  willemite, 
and  Balmain's  paint.  Characteristic  curves  for  these  three  substances, 
together  with  a  representative  curve  for  Sidot  blende,  are  shown  together 
in  Fig.  59.  It  will  be  noticed  that  the  curves  are  all  of  the  same  type.  In 
each  case  the  decay  is  at  first  rapid  and  apparently  according  to  the  same 
law  that  was  found  to  hold  during  the  early  stages  of  decay  in  the  case  of 
Sidot  blende.  After  20  or  30  seconds  the  curves  begin  to  bend,  and  finally 
become  straight  lines  whose  slant  is  less  than  that  of  the  earlier  part  of  the 
curve. 

Upon  plotting  the  data  of  E.  Becquerel2  in  the  same  manner  we  find  that 
in  several  cases  the  curves  are  of  exactly  the  same  type  as  those  obtained 
by  us.  Three  such  curves  are  shown  in  Fig.  60.  In  fact,  all  the  data 
recorded  by  Becquerel  in  his  papers  on  this  subject  give  curves  which 
show  the  same  general  characteristics,  although  in  several  instances  the 
curves  are  not  so  smooth  as  those  shown  in  this  figure. 


'Obtained  from  Leppin  and  Masche,  who  do  not  state  the  composition  of  the  powder.  The  chief  constit- 
uent is,  however,  zinc  sulphide,  so  that  the  substance  appears  to  be  a  modified  form  of  Sidot  blende.  It  is 
said  to  be  especially  sensitive  to  the  influence  of  the  radio-active  emanations. 

"Becquerel,  La  Lumiere.     Also  Annales  de  Chimie  et  de  Physique,  Series  3,  62,  1861. 


68 


STUDIES   IN   LUMINESCENCE. 


It  is  interesting  to  note  also  that  the  data  of  Darwin1  on  Balmain's  paint 
give  a  curve  similar  to  those  obtained  by  us  with  this  substance  when 
plotted  in  the  same  way.  Two  decay  curves  taken  by  E.  Wiedemann2  with 
Balmain's  paint  also  show  the  same  characteristics. 


Fig.  59- 

Typical  decay  curves  for  various  substances.     Curve  A,  Sidot  blende;  curve  B,    "Emanations-pulver;" 
curve  C,  Willemite;  curve  D,  Balmain's  paint. 

It  appears  therefore  that  the  decay  curve  for  Sidot  blende  is  similar  in 
its  main  features  to  the  curves  obtained  for  a  large  number  of  other  phos- 
phorescent substances.  In  fact,  we  do  not  know  of  any  case  of  long-time 
phosphorescence  in  solids  which  shows  a  different  type  of  curve. 


Fig.  60. 

Decay  curves  plotted  from  the  data  of  E. 

Becquerel.      (Ann.  de  Chimie  et  de 

Physique,  series  3,  vol.  62,  1861.) 
Curve  A,  Sulfure  de  calcium  lumineux 

vert  (2,  p.  69). 
Curve  B,  Sulfure  de  strontium  lumineux 

vert  (p.  71). 
Curve    C,  Sulfure]  jaune-orange  vif  (D 

68). 


40O  60O  800 

Seconds 


The  peculiar  behavior  of  Sidot  blende  which  we  have  compared  with 
magnetic  hysteresis  is  also  exhibited  by  willemite  and  Balmain's  paint,  as 
illustrated  in  Figs.  61  and  62.  In  each  of  these  cases  it  is  evident  that  the 
effect  of  a  given  excitation  is  dependent  on  the  previous  history  of  the  sub- 
stance. We  have  not  yet  had  an  opportunity  to  test  this  phenomenon  in 
other  substances. 


Darwin,  Philosophical  Magazine,  n,  p.  209,  1881. 

»E.  Wiedemann,  2ur  Mechanik  der  Leuchtens,  Wied.  Ann.,  37,  p.  177,  1889. 


DECAY   OF   PHOSPHORESCENCE    IN    SIDOT   BLENDE. 


69 


SUMMARY. 

The  most  important  points  brought  out  by  the  experiments  here  de- 
scribed may  be  briefly  stated  as  follows : 

i.  Form  of  Decay  Curve. — The  curve  obtained  by  plotting  the  values  of 
7~*  as  ordinates  and  the  corresponding  values  of  /  as  abscissas  is  a  straight 
line  for  small  values  of  t ;  it  changes  to  a  curve  concave  toward  the  axis  of  t 
as  /  increases;  but  for  still  larger  values  of  /  the  relation  between  7~*  and 


10  20  30  40  50  60 


Fig.  6 1.— Willemite. 

Excited  by  a  spark  between  iron 
terminals.  The  curves  were  taken 
in  the  order  indicated  by  the  let- 
ters and  with  the  excitations 
stated  below: 

Curve  A,  0.7  sec.;  B,  3.4  sec.;  C, 
9.0  sec.;  D,  60.0  sec.;  E,  3.9  sec.: 
F,  i.o  sec. 


/vT 


Fig.  62. — Balmain's  paint. 

The  curves  were  taken  in  the  order 
indicated  by  the  letters,  and  with 
the  following  excitations: 

Curve  A,  5.9  sec.;  B,  12.0  sec.;  C, 
26.0  sec.;  0,42.0  sec.;  E,  6.4  sec. 


100  200  300  400 

Seconds 

/  is  again  linear,  and  remains  so  until  7  becomes  too  small  to  measure.  In 
other  words,  the  decay  curve,  when  plotted  in  this  way,  consists  of  two 
straight  portions  which  gradually  merge  into  one  another. 

2.  Effect  of  Duration  and  Intensity  of  Excitation. — Not  only  the  intensity 
of  phosphorescence,  but  also  the  form  of  the  decay  curve,  is  dependent  on 
the  intensity  and  duration  of  excitation.  The  slant  is  altered  in  each  of 
the  straight  parts  of  the  curve  by  changing  either  of  these  two  factors  in 
the  excitation. 


7O  STUDIES  IN  LUMINESCENCE. 

3.  Hysteresis. — The  behavior  of  the  phosphorescent  substance  with  a 
given  excitation  depends  upon  its  previous  history.     Some  semi -permanent 
change  is  produced  by  excitation  which  persists  for  several  hours,  or  even 
for  several  days,  after  visible  phosphorescence  has  ceased. 

4.  Effect  of  Red  and  Infra-red  Rays. — In  the  case  of  Sidot  blende  the 
semi-permanent  condition  produced  by  excitation  may  be  destroyed  and 
the  screen  restored  to  a  standard  state  by  brief  exposure  to  the  red  and 
infra-red  rays. 


CHAPTER  V. 

THE  INFLUENCE  OF  THE  RED  AND  INFRA-RED  RAYS  UPON 
THE  PHOTO-LUMINESCENCE  OF  SIDOT  BLENDE.' 

The  effect  of  the  red  and  infra-red  rays  in  suppressing  the  phosphorescence 
of  various  substances  has  long  been  known,2  and  has  frequently  been 
utilized  in  the  study  of  the  infra-red  spectrum.  The  effect  is  exhibited 
by  Sidot  blende  more  strongly  perhaps  than  by  any  of  the  other  phos- 
phorescent sulphides.  In  Chapter  IV  we  have  called  attention  to  another 
effect  produced  by  the  longer  waves,  namely,  the  restoration  of  a  screen  of 
Sidot  blende,  after  the  excitation  and  complete  decay  of  phosphorescence, 
to  a  standard  condition,  so  that  the  result  of  a  subsequent  excitation  shall 
be  unaffected  by  the  previous  history  of  the  substance.  While  this  new 
effect  is  doubtless  connected  in  some  way  with  that  first  mentioned,  the 
nature  of  the  relationship  between  the  two  is  by  no  means  clear.  For  this 
reason,  and  because  of  the  bearing  of  the  phenomena  upon  the  general 
theory  of  luminescence,  we  have  investigated  the  influence  of  the  longer 
waves  upon  the  luminescence  of  Sidot  blende  under  a  variety  of  different 
conditions. 

The  work  naturally  falls  under  several  heads,  as  follows: 

1 .  The  effect  upon  the  luminescence  of  Sidot  blende  of  exposure  to  the 
longer  waves  before  excitation. 

2.  The  effect  of  the  longer  waves  during  excitation. 

3.  The  effect  of  the  longer  waves  after  excitation,  i,  e.,  during  the  decay  of 
phosphorescence. 

4.  The  influence  upon  the  effect  studied  of  the  wave-length  of  the  red 
and  infra-red  rays  used. 

THE  EFFECT  OF  THE  LONGER  WAVES  BEFORE  EXCITATION. 

Experiments  described  in  the  preceding  chapter  indicate  that  when  Sidot 
blende  is  excited  to  luminescence  "  some  change  is  produced  in  the  material 
by  the  action  of  the  exciting  light,  and  that  this  change  persists  for  a  con- 
siderable period  after  a  visible  phosphorescence  has  ceased.  In  other  words, 
the  effect  of  a  given  excitation  in  producing  phosphorescence  depends  upon 
the  previous  history  of  the  phosphorescent  substance.  If  the  screen  is 
allowed  to  rest  in  the  dark  for  a  number  of  hours  this  semi-permanent  effect 
of  exposure  in  part  dies  out.  But  rest  alone  does  not  restore  the  screen 
completely  even  if  continued  for  several  days."  An  exposure  of  a  few 
seconds  to  the  rays  from  a  50  c.  p.  lamp  seen  through  ruby  glass  is,  however, 
sufficient  to  restore  the  screen  to  what  seems  to  be  a  definite  standard 
condition. 


'The  substance  of  this  chapter  essentially  as  here  given  appeared  in  the  Physical  Review,  xxv,  p.  362. 
^References  to  the  literature  of  the  subject  are  given  by  Dahms,  Annalen  der  Physik,  13,  p.  425. 

71 


STUDIES  IN  LUMINESCENCE. 


The  phenomenon  in  question  is  illustrated  by  the  curves  of  Fig.  50,  p.  62. 
If  we  compare  curves  A,  C,  and  D,  all  corresponding  to  the  same  excitation, 
it  is  clear  that  exposure  to  the  longer  waves  before  excitation  exerts  a  very 
marked  influence  upon  the  rate  at  which  the  phosphorescence,  excited  after 
this  exposure,  will  decay.  While  the  semi-permanent  change  produced 
by  excitation  is  partly  lost  as  the  result  of  prolonged  rest  in  the  dark,  rest 
alone  is  not  a  very  satisfactory  means  of  restoring  the  substance  to  a  stand- 
ard condition.  Thus  a  rest  of  24  hours  brings  about  a  change  in  the  decay 
curve  following  a  lo-second  excitation  from  C  to  A .  Rest  for  several  days 
would  shift  the  curve  somewhat  farther  to  the  left.  But  even  a  rest  of 
several  weeks  failed  to  bring  the  decay  curve  as  far  to  the  left  as  curve  D. 

Curve  D,  Fig.  50,  was  taken  after  an  exposure  of  4  minutes  to  the  longer 
waves.  A  very  much  shorter  exposure  would  have  been  nearly,  if  not  quite, 
as  effective.  This  point  is  brought  out  by  the  curves  of  Fig.  63,  which  were 
taken  to  determine  the  way  in  which  the  effect  depended  upon  the  duration 
of  exposure  to  the  longer  waves.  In  taking  these  curves  the  procedure  was 
as  follows :  In  each  case  the  screen  was  first  exposed  for  2  minutes  to  the 


Fig.  63. 

Effect  on  the  decay  curve  of  exposure  to  the  infra- 
red for  different  times. 

Curve  /,  i  o  seconds  exposure  after  48  hours  rest  in  the 
dark;  //,  2  minutes  exposure;  ///,  ip  seconds  ex- 
posure after  i  second  infra-red;  71",  10  seconds 
exposure  after  3  seconds  infra-red;  !•',  10  seconds 
exposure  after  60  seconds  infra-red. 


40  60  BO  $»(.    |00 

mercury  arc ;  the  phosphorescence  was  then  allowed  to  decay  for  2  minutes, 
at  the  end  of  which  time,  while  still  visible,  it  was  too  faint  for  measurement. 
The  screen  was  then  exposed  to  the  rays  of  a  50  c.p.  lamp  at  a  distance  of 
5  inches,  a  piece  of  ruby  glass  being  interposed  between  the  lamp  and  the 
screen ;  the  duration  of  this  exposure  was  i  second  for  curve  ///,  3  seconds 
for  curve  /  V,  and  60  seconds  for  curve  V.  After  this  exposure  to  the  longer 
waves  the  screen  was  excited  by  the  mercury  arc  for  10  seconds,  and  the 
decay  curves  shown  in  Fig.  63  were  observed  by  the  procedure  described 
on  page  58. 

It  will  be  observed  that  even  an  exposure  of  only  i  second  is  more  effec- 
tive than  48  hours  rest.  It  will  be  noticed  also  that  3  seconds  exposure  to 
the  longer  waves  is  nearly  as  effective  as  an  exposure  of  a  minute. 

With  red  and  infra-red  rays  of  less  intensity  a  longer  time  is  required 
to  destroy  the  effect  of  previous  excitation.  The  curves  of  Fig.  64  were 
taken  with  a  procedure  similar  to  that  for  Fig.  63,  except  that  the  distance 
of  the  50  c.p.  lamp  from  the  Sidot  blende  screen  was  30  inches  instead  of  5 
inches.  Exposure  for  60  seconds  to  these  less  intense  rays  produces  as 


INFLUENCE   OF   RED  AND   INFRA-RED   RAYS   UPON   SIDOT   BLENDE-       73 

great  an  effect  as  a  similar  exposure  to  the  stronger  rays.  But  this  is  not 
true  for  shorter  exposures.  While  the  ultimate  effect  of  the  weaker  rays 
is  apparently  the  same,  more  time  is  required  to  produce  the  change  when 
the  rays  are  of  small  intensity;  approximately,  at  least,  the  change  produced 
depends  upon  the  product  of  intensity  and  duration  of  exposure. 

The  experimental  data  bearing  upon  this  phase  of  the  subject  are  so 
meager  as  to  permit  of  only  the  most  general  conclusions,  and  additional 
experiments  are  much  to  be  desired.  So  far  as  they  go,  however,  the  results 
indicate  that  the  condition  in  which  the  material  is  left  after  the  excitation 
and  decay  of  phosphorescence  is  an  unstable  one,  due  perhaps  to  some  new 
grouping  of  the  molecules  of  the  phosphorescent  material.  During  rest  in 
the  dark  accidental  disturbances  of  various  kinds  may  cause  the  substance 
to  return  more  or  less  completely  to  its  normal  condition.  The  effect  of 
rest  is  therefore  uncertain,  depending  as  it  does  upon  the  extent  to  which 
various  obscure  and  perhaps  unrecognized  agencies  are  active ;  but  certain 
waves  lying  chiefly  in  the  infra-red  region  of  the  spectrum  have  a  definite 
and  positive  effect  in  restoring  the  substance  to  its  normal  condition. 


Fig.  64. 

Effect  on  the  decay  curve  of  exposure  to  the  infra- 
red for  different  times.  The  intensity  of  the 
infra-red  rays  was  here  only  about  j'g  of  the  inten- 
sity used  for  the  curves  of  Fig.  63. 

Curve/,  10  seconds  exposure  after  48  hours  rest  in 
the  dark;  //,  2  minutes  exposure ;  ///,  10  seconds 
exposure  after  60  seconds  infra-red;  IV.  10  seconds 
exposure  after  15  seconds  infra-red. 


00     See.  I0> 

INFLUENCE   OF  THE   LONGER  WAVES  DURING   EXCITATION. 

We  have  seen  that  a  condition  is  developed  in  Sidot  blende  by  excitation 
which  is  favorable  to  the  production  of  strong  luminescence  by  a  subsequent 
excitation.  A  long  excitation  is  therefore  more  effective  than  exposure 
to  equally  intense  exciting  rays  for  a  shorter  period ;  for  the  favorable  con- 
dition developed  in  the  early  stages  of  excitation  makes  the  exciting  rays 
that  act  later  more  effective.  The  luminescence  of  such  a  substance  during 
excitation — /.  e.,  the  fluorescence — will  be  relatively  weak  when  excitation 
first  begins  and  will  increase  in  intensity  as  the  exposure  continues,  reaching 
a  steady  value  only  after  a  considerable  time.  In  Sidot  blende,  with  the 
exciting  light  used  in  most  of  our  experiments,  3  or  4  minutes  were  required 
to  reach  a  steady  value.1 

The  steady  condition  finally  reached  is  manifestly  characterized  by 
equality  in  the  rates  of  development  and  decay  of  the  condition  favorable 
to  luminescence  to  which  we  have  just  referred.  If  we  think  of  this  favor- 


>See  the  preceding  chapter  of  this  memoir. 


74  STUDIES  IN   LUMINESCENCE. 

able  condition  as  being  due  to  some  new  grouping  of  the  molecules,  then 
the  condition  of  steady  fluorescence  is  reached  when  these  favorable  groups 
are  being  broken  up,  either  spontaneously  or  through  the  action  of  some 
outside  agent,  just  as  rapidly  as  they  are  formed  by  the  action  of  the  excit- 
ing light. 

It  is  clear  that  the  intensity  of  steady  fluorescence  will  be  made  less  by 
any  agent  which  increases  the  rate  at  which  the  assumed  favorable  group- 
ing is  destroyed.  Now  it  is  precisely  this  effect  that  is  exerted  by  the 
red  and  infra-red  rays;  and  we  should  therefore  anticipate  that  the  fluo- 
rescence of  Sidot  blende  would  be  diminished  by  the  action  of  longer  rays. 

This  effect  of  the  longer  waves,  which  does  not  appear  to  have  attracted 
much  attention  heretofore,  may  readily  be  made  very  marked  indeed. 
Thus  the  rays  from  a  projecting  lantern  after  passing  through  a  sheet  of 
hard  rubber  0.2  mm.  thick  are  able  to  reduce  the  fluorescence  of  Sidot  blende 
so  greatly  as  to  leave  the  intensity  only  a  few  per  cent  of  its  normal  value, 
and  this  too  with  very  intense  excitation.  If  the  ultra-violet  rays  of  a 
spark  are  used  for  excitation  numerous  lecture  experiments  may  be  devised 
for  demonstrating  the  existence  of  the  invisible  rays  at  the  two  ends  of  the 
spectrum.  As  compared  with  the  experiments  first  proposed  by  Dahms,1 
in  which  the  effect  of  the  infra-red  rays  upon  phosphorescence  is  utilized, 
this  procedure  has  the  advantage  of  giving  a  persistent  rather  than  a  fleet- 
ing effect. 

In  studying  the  influence  of  the  longer  rays  upon  fluorescence  we  have 
directed  our  attention  especially  to  the  distribution  of  the  effect  through- 
out the  fluorescence  spectrum.  While  the  result  of  exposure  to  longer 
waves  is  to  diminish  greatly  the  total  brightness  of  the  fluorescence  light, 
it  might  be  that  in  certain  restricted  regions  of  the  spectrum  the  intensity 
would  be  increased  rather  than  diminished,  or  at  least  that  the  effect  of  the 
infra-red  rays  would  vary  greatly  in  magnitude  in  different  parts  of  the 
fluorescence  spectrum. 

For  the  study  of  this  phase  of  the  subject  the  Sidot  blende  screen  was 
mounted  in  front  of  a  Lummer-Brodhun  spectrophotometer,  with  an  acety- 
lene flame  as  a  comparison  source  as  in  our  previous  work.  The  infra- 
red rays  from  an  arc  fell  upon  the  screen  after  passing  through  hard  rubber. 
The  intensity  of  fluorescence  was  then  measured  in  different  parts  of  the 
spectrum,  first  with  and  then  without  the  action  of  the  longer  waves,  the 
excitation  remaining  constant.  As  exciting  source  a  mercury- vapor  lamp 
was  first  used,  the  lamp  being  made  of  the  so-called  "Uviol"  glass,  which 
possesses  an  unusual  transparency  to  the  ultra-violet  rays. 

In  Fig.  65,  curve  7,  shows  the  ordinary  fluorescence  spectrum  of  Sidot 
blende  produced  by  the  mercury  lamp,  while  curve  II  shows  the  spectrum 
as  modified  by  exposure  to  the  infra-red.  In  the  case  of  curves  /'  and  //'  a 
sheet  of  ordinary  glass  was  interposed  between  the  lamp  and  the  screen,  so 
that  the  ultra-violet  rays  were  in  large  part  removed  from  the  exciting  light. 
It  is  clear  that  the  ultra-violet  rays  of  the  Uviol  lamp  introduce  a  band 
at  about  0.49  /z  which  overlaps  and  distorts  the  usual  green  band  at  0.51  //. 

A  more  annoying  source  of  disturbance  in  these  experiments,  however, 
was  the  light  reflected  from  the  screen,  which  was  mixed  with  fluorescence 

'Dahms.  /.  C. 


INFLUENCE   OF   RED   AND   INFRA-RED   RAYS   UPON   SIDOT   BLENDE.        75 


light  and  practically  inseparable  from  it.     By  making  observations  only 
at  points  lying  between  the  bright  lines  of  the  mercury  spectrum  we  had 


Fig.  65. 

Effect  of  infra-red  on  fluorescence.     Sidot  blende  excited  by 

"  Uviol"  mercury-vapor  lamp. 
Curve  /.  Fluorescence  spectrum. 
Curve  //.  Fluorescence  spectrum  when  screen  is  exposed  to 

infra-red  during  excitation. 
Curve  /'.  Fluorescence  spectrum  with  plate  glass  between  screen 

and  mercury  lamp. 
Curve  //'.  Same  as  /',  except  that  screen  is  also  exposed  to 

infra-red. 
It.    Reflection  of  exciting  light  from  white  surface. 


-44    M     .48     .50     .X     -54     .56/4 


expected  to  be  untroubled  by  reflected  light.  But  owing  either  to  optical 
imperfections  in  the  apparatus  or  to  the  existence  of  a  faint  continuous 
spectrum  in  the  light  from  the  lamp,  there  was  I5| 
always  enough  reflected  light  in  the  field  of  the 
spectrophotometer  to  be  an  important  and  dis- 
turbing factor.  To  get  some  idea  of  the  inten- 
sity and  distribution  of  this  reflected  light  we 
made  the  observations  plotted  as  curve  R  in  Fig. 
65  with  a  screen  of  MgO  on  cardboard  instead 
of  the  Sidot  blende  screen.  To  avoid  confusion 
this  curve  is  displaced  downward  in  the  plot. 
The  intensity  for  points  on  curve  R  should  be 
read  from  the  right-hand  side  of  the  figure. 

The  irregular  distribution  of  the  reflected 
light  and  the  great  uncertainty  in  its  measure- 
ment make  the  experiments  plotted  in  Fig.  65 
of  little  quantitative  value.  Especially  is  this 
true  for  the  violet  end  of  the  spectrum,  where 
the  reflected  rays  are  of  great  intensity.  We 
could  not  even  feel  sure  that  the  longer  waves 
produced  any  effect  at  all  in  this  region. 

With  a  different  zinc  sulphide  screen,  the 
so-called  "  Emanations-pulver "  referred  to  in 
Chapter  IV,  the  conditions  were  somewhat 
more  favorable.  The  curves  in  Fig.  66  show 
the  ordinary  fluorescence  spectrum  (curve  7); 
the  fluorescence  spectrum  with  exposure  to 
weak  infra-red  rays  (curve  77) ;  the  fluorescence 
spectrum  during  exposure  to  strong  infra-red 
rays  (curve  777) ;  and  the  reflected  light  deter- 
mined as  before.  Different  intensities  of  infra-red  were  obtained  by  using 
in  one  case  one  piece  of  black  rubber  and  in  the  other  case  two  pieces 
between  the  arc  lamp  and  the  screen.  By  this  procedure  it  is  possible  to 


/  \ 


\ 


III 


12 


10 


,46    .48    .£0  ..52    .34  ~5 % 

Fig.  66. 

Effect  of  infra-red  on  fluorescence 
of  "Emanations-pulver,"  ex- 
cited by  Uviol  lamp. 


STUDIES  IN   LUMINESCENCE. 


\ 


A 


V 


determine  the  ratio  of  the  effects  produced  by  strong  and  weak  infra-red, 
in  spite  of  the  uncertainty  in  the  value  of  the  reflected  light.  Thus  the 
difference  between  the  ordinate  of  curve  /  and  the  corresponding  ordinate 
of  curve  //  is  a  measure  of  the  effect  produced  by  weak  infra-red ;  reflected 
light,  since  it  affects  both  measurements,  is  eliminated  by  taking  their  differ- 
ence. Similarly  the  difference  between  the  ordinates  of  curves  7  and  777 
measures  the  effect  of  the  stronger  infra-red.  It  is  interesting  to  note  that 
the  ratio  of  these  effects  is  nearly  constant  throughout  the  green  band. 
Beginning  at  0.562  ju  and  running  toward  the  violet  the  ratio  has  the  values : 
1.32,  1.36,  1.31,  1.22,  1.94.  Except  for  the  last  point,  which  is  so  near  the 
edge  of  the  band  that  the  intensity  of  fluorescence  is  small,  the  values  are 
constant  to  within  observational  errors.  This  fact  adds  another  to  the 

many  that  have  been  observed  in  the  course  of  our 

..    % work  on  luminescence  to  indicate  that  each  band 

in  a  luminescence  spectrum  behaves  as  a  unit — 
that  whatever  affects  one  part  of  the  band  affects 
all  other  parts  of  the  band  in  the  same  proportion. 
A  more  satisfactory  method  of  studying  the 
effect  in  question  is  to  use  only  ultra-violet  light 
in  excitation.  All  troubles  due  to  reflected  light 
are  in  this  case  removed.  The  results  of  this  pro- 
cedure in  the  case  of  the  original  Sidot  blende  used 
in  our  earlier  experiments  are  shown  in  Fig.  67. 
An  iron  spark  was  used  as  an  exciting  source,  a 
spectrum  being  formed  by  a  quartz  train  and  only 
the  ultra-violet  rays  used.  The  source  of  infra- 
red was  an  arc  lantern  whose  rays  passed  through 
a  sheet  of  hard  rubber  0.2  mm.  thick.  The  effect 
of  exposure  to  longer  rays  during  excitation  by  the 
ultra-violet  rays  of  the  iron  spark  was  to  change 
the  fluorescence  from  curve  7  to  curve  77.  The 
diminution  in  intensity  brought  about  by  exposure 
to  the  longer  rays,  expressed  as  a  fraction  of  the 
ordinary  fluorescence  at  the  same  wave-length,  is 
given  in  curve  777. 

More  extended  experiments  are  required  to  determine  whether  the  change 
in  the  effect  from  38  per  cent  at  0.546  /j,  to  20  per  cent  at  0.480  n  is  real  or 
the  result  of  errors.  It  does  not  seem  likely,  however,  that  experimental 
errors  alone  can  account  for  so  great  a  change.  In  interpreting  the  results 
we  must  bear  in  mind  the  fact  that  the  spectrum  shown  in  Fig.  67  obviously 
consists  of  two  overlapping  bands,  and  that  the  infra-red  effect  may  differ 
for  the  two.  It  is  highly  probable  also  that  still  another  band  is  present 
at  0.49  n,  as  was  found  to  be  the  case  with  ultra-violet  excitation  in  the 
experiments  plotted  in  Fig.  65 ;  and  for  this  band  the  effect  of  the  longer 
waves  may  be  different  still.  It  is  clear  that  further  experiments  on  this 
branch  of  the  subject  are  needed. 

EFFECT   OF  THE   LONGER  WAVES  DURING   DECAY. 

In  studying  the  effect  of  the  infra-red  rays  upon  the  decay  of  phosphores- 
cence two  methods  were  used.  In  the  first  of  these  the  intensity  of  the 


Fig.  67. 

Effect  of  infra-red  upon  flupres- 
cence  of  "  Emanations- 
pulver"  excited  by  ultra- 
violet rays  of  an  iron  spark. 


INFLUENCE   OF   RED   AND   INFRA-RED   RAYS   UPON   SIDOT   BLENDE.        77 

total  phosphorescent  light  was  measured  by  a  photometer  at  different 
times  after  excitation  had  ceased,  as  described  in  Chapter  IV.  The  violet 
end  of  the  carbon  arc  spectrum  was  used  for  excitation,  and  a  50  c.p. 
incandescent  lamp  as  a  source  of  infra-red  rays.  In  these  experiments  a 
cell  containing  a  solution  of  iodine  in  carbon  disulphide  was  used  instead 
of  hard  rubber  to  remove  the  visible  rays.  The  distance  of  the  lamp 
from  the  Sidot  blende  screen  was  about  60  cm. 

The  curves  of  Figs.  68  and  69  show  some  of  the  results  obtained  by  this 
procedure.  In  Fig.  68,  curve  /  is  the  ordinary  decay  curve  without  expo- 
sure to  infra-red.  In  the  case  of  curve  ///  the  infra-red  rays  were  allowed 
to  fall  on  the  screen  when  the  decay  had  proceeded  for  about  32  seconds. 
In  curve  //  the  infra-red  rays  were  turned  on  about  4  seconds  after  the 


^J 


Fig.  68. 

I  nfluence  of  exposure  to  infra-red 
during  decay  upon  the  form  of 
the  decay  curve. 

Curve/.  Ordinary  decay  curve. 

Curve  //.  Screen  exposed  to 
infra-red  after  decay  had  pro- 
ceeded for  4  seconds.  Infra- 
red cut  off  at  /  =  19  seconds. 

Curve  ///.  Screen  exposed  to 
infra-red  after  decay  had  pro- 
ceeded for  32  seconds. 


40 


80 


100 


sec.  140 


end  of  excitation  and  were  cut  off  again  at  the  end  of  about  19  seconds. 
The  great  increase  in  the  rapidity  of  decay  brought  about  by  infra-red  rays, 
even  when  of  such  small  intensity  as  those  used  in  these  experiments,  is 
clearly  shown. 

There  was  no  indication  in  these  experiments  of  any  temporary  increase 
in  the  brightness  of  phosphorescence  when  the  rays  were  first  turned  on, 
as  has  been  noted  by  many  observers  in  the  case  of  Balmain's  paint  and 
other  phosphorescent  sulphides.  Dahms  has  already  called  attention  to 
this  peculiarity  of  Sidot  blende.  The  effect  of  the  longer  waves  in  suppress- 
ing phosphorescence  has  sometimes  been  explained  by  assuming  that  these 
rays  act  in  the  same  way  as  does  a  rise  of  temperature;  i.  e.,  that  they 
accelerate  the  process  which  causes  phosphorescence,  so  as  to  produce  a 
brief  flash,  due  to  the  sudden  liberation  of  the  energy  stored  during  excita- 
tion, followed  by  a  complete  loss  of  luminescence  when  the  stored  energy 
has  been  used  up.  While  this  explanation  of  the  phenomenon  may  be 
correct  for  the  other  phosphorescent  substances  it  can  not  be  applied  with- 
out essential  modification  to  the  case  of  Sidot  blende.  It  has  recently  been 
shown,  however,  by  Ives  and  Luckiesh1  that  under  some  circumstances  a 

"Physical  Review,  xxxn,  p.  240,  1911.  The  specimen  of  Sidot  blende  that  was  tested  by  Ives  and 
Luckiesh  differed  from  that  used  by  us  and  other  observers  in  the  fact  that  the  law  of  decay  after  one  minute 
was  /~°'»7  =  o+6/  instead  of  /  —  W  =  0+W.  It  thus  forms  the  only  exception  known  to  us  to  the  law  of  de- 
cay discussed  in  Chapter  XV. 


STUDIES   IN   LUMINESCENCE. 


flash  may  be  observed  immediately  after  exposure  to  infra-red  rays  even 
in  the  case  of  Sidot  blende.  These  observers  find  that  "there  are  two  stages 
in  the  decay  of  phosphorescence,  merging  gradually  one  into  the  other.  In 
the  first  stage  the  effect  of  long  waves  is  to  cause  a  sudden  drop  in  intensity ; 
in  the  second  stage  the  long  waves  cause  a  temporary  increase  of  brightness 
before  decay.  In  the  intermediate  stage  the  accelerated  decay  appears  to 
the  unaided  eye  to  be  delayed  in  starting."  With  intense  infra-red  rays 
the  preliminary  flash  could  be  made  to  occur  as  early  as  30  seconds  after 
the  end  of  excitation. 

Perhaps  the  most  striking  feature  of  the  curves  in  Fig.  68  is  the  nearly 
exact  parallelism  that  exists  between  the  later  part  of  curve  ///  (after  the 
infra-red  rays  were  cut  off)  and  the  straight  part  of  the  ordinary  decay 
curve.  The  action  of  the  longer  waves  appears  to  be  to  bring  the  material 
quickly  into  the  same  condition  as  regards  ability  to  emit  light  that  it 
would  have  acquired  at  the  end  of  a  much  longer  period  of  ordinary 
decay.  The  results  of  Ives  and  Luckiesh  are  not  in  accord  with  this  view. 

'Si 


Fig.  69. 

Influence  of  exposure  to  infra-red  during  decay 
upon  the  form  of  the  decay  curve. 

Curve  /.  Ordinary  decay  curve. 

Curve  //.  Screen  exposed  to  infra-red  after  the 
decay  had  proceeded  for  18  seconds. 

Curve  ///.  Screen  exposed  to  infra-red  during  exci- 
tation and  for  first  16  seconds  of  decay. 

Curve  IV.  Screen  exposed  to  infra-red  during  exci- 
tation and  for  first  9  seconds  of  decay. 


«0  40  60  60  5*c.    108 

In  the  brief  abstract  of  their  work  referred  to  on  page  77  they  state  that 
"the  decay  curve  after  brief  action  of  red  or  infra-red  does  not  correspond 
with  the  original  curve  with  origin  shifted." 

In  the  case  of  curves  ///  and  I V  of  Fig.  69  the  screen  was  exposed  to 
infra-red  during  excitation  and  during  the  early  stages  of  decay.  The 
infra-red  rays  were  cut  off  at  the  points  indicated  by  the  break  in  the  curves. 
It  will  be  noticed  that  the  latter  portion  of  curve  ///  is  nearly  parallel  to  the 
straight  part  of  the  ordinary  decay  curve;  but  in  curve  IV,  where  the  infra- 
red was  cut  off  earlier,  the  straight  part  of  the  curve  is  not  even  approxi- 
mately parallel  to  curve  7.  The  results  shown  in  Fig.  69  are  thus  in  agree- 
ment with  the  statement  of  Ives  and  I/uckiesh. 

For  the  early  stages  of  decay  a  number  of  curves  were  taken  by  means 
of  the  spectrophotometer,  the  method  being  that  described  in  Chapter  III. 
With  this  method  it  is  impracticable  to  follow  the  decay  for  more  than  a 
few  seconds,  since  the  illumination  of  the  spectrophotometer  field  soon 
becomes  too  faint  for  accurate  measurements.  The  method  possesses  a 
great  advantage,  however,  in  the  fact  that  the  effect  of  the  long  waves  can 
be  determined  for  different  parts  of  the  phosphorescence  spectrum. 


INFLUENCE;  OF  RED  AND  INFRA-RED  RAYS  UPON  SIDOT  BLENDE.     79 


For  the  curve  shown  in  Figs.  70  and  7 1  the  exciting  light  was  the  violet 
of  the  carbon  arc  spectrum.  For  the  curves  of  Figs.  72  to  75  a  spark  between 
cadmium  terminals  was  used  in  excitation.  The  intensity  of  phospho- 
rescence has  been  plotted  in  all  of  these  curves  instead  of  the  reciprocal 
square  root,  although  in  some  cases  the  latter  value  has  been  plotted  also. 


80 


to 


Fig.  70. 

Influence  of  exposure  to  infra- 
red on  form  of  decay  curve. 

Curve  7.  Ordinary  decay  curve 
for  X  =0.497  /t. 

Curve  /'.  Screen  exposed  to 
infra-red  immediately  after 
end  of  excitation. 


6  sec.? 


(00 

I 

60 


60 


to 


\ 


V 


Fig.  7i. 

Curve  /.  Ordinary  decay  curve  for  X  =0.497  /»• 
Curve  /'.  Screen  exposed  to  infra-red  after  decay 
had  proceeded  to  A. 


a*    Sec.  & 


10 


Fig.  72. 

Decay  curves  with  and  with- 
out infra-red.    X  =0.5460. 


8o 


STUDIES  IN  LUMINESCENCE. 


-so 


30 


10 


k 


In  all  these  figures  the  curve  marked  7  was  taken  without  infra-red1  and 
that  marked  7'  with  infra-red.  In  general  the  exposure  to  infra-red  began 
at  the  instant  the  excitation  ceased,  the  shutter  being  arranged  so  that  the 
same  movement  that  cut  off  the  exciting  rays  allowed  the  infra-red  rays  to 
fall  upon  the  screen.  In  the  case  of  Fig.  71  exposure  to  the  infra-red  did 
not  begin  until  about  1.4  seconds  after  the  end  of  excitation.  Numerous 
curves  of  this  kind  were  taken  in  which  the  exposure  to  infra-red  began  at 
different  times  after  the  beginning  of  decay.  All  of  these  curves  show  the 
same  sudden  drop  in  intensity  at  the  instant  that  the  long  waves  begin  to 
act. 

The  curves  of  Figs.  70,  71,  and  72,  except  for  the  method  of  plotting,  are 
quite  similar  to  the  curves  for  the  total  light  obtained  by  the  photometer 
method  first  described.  Apparently  the  action  of  the  longer  waves  during 
the  first  few  seconds  of  decay  is  quite  similar  to  its  action  later.  It  will 

be  observed  that  the  curves  of  Figs. 
70  to  72  refer  to  regions  of  the  phos- 
phorescence spectrum  either  near 
the  maximum  or  toward  the  red 
edge  of  the  band. 

Figs.  73,  74,  and  75  show  the  in- 
fluence of  the  longer  waves  upon 
those  regions  of  the  fluorescence 
spectrum  lying  near  and  beyond  the 
violet  edge  of  the  green  band.  Here 
the  effect  seems  to  be  entirely  dif- 
ferent. At  0.445  M  (Fig-  75)  the 
effect  of  exposure  to  infra-red  is  to 
retard  the  decay  of  phosphorescence 
instead  of  to  accelerate  it.  In  the 
region  lying  between  the  green  band 
(0.51  /z)  and  the  violet  band  (0.45  /JL) 
the  effect  of  the  infra-red  is  at  first 
to  retard  the  decay  and  later  to  ac- 
celerate it  (Figs.  73  and  74). 

Owing  to  the  faintness  of  the  spec- 
trophotometer  field  and  the  rapidity 

with  which  the  phosphorescence  decays  it  is  difficult  to  determine  the  form 
of  the  curves  in  the  early  stages  of  decay  with  accuracy.  Especially  is  this 
true  in  the  blue  and  violet,  owing  to  the  small  luminosity  of  this  region  of 
the  spectrum.  Each  point  plotted  represents,  however,  the  average  of  a 
number  of  separate  readings.  For  each  pair  of  points  the  observations 
for  the  time  of  decay  with  and  without  infra-red  were  taken  alternately. 
No  special  precautions  were  taken  to  keep  the  exciting  source  constant. 
The  slight  initial  curvature  of  the  line  7~*  in  Figs.  70  and  73,  where  the 
exciting  light  was  from  the  carbon  arc,  is  perhaps  to  be  explained  as  the 
result  of  variations  in  this  source.  In  the  case  of  the  other  observations,  in 
which  a  spark  was  used  in  excitation,  the  points  for  7~*  lie  reasonably  well 

>The  carbon  arc  was  used  as  a  source  of  infra-red,  a  piece  of  dense  ruby  glass  serving  as  a  filter.  Red 
light,  and  perhaps  a  little  yellow  light,  was  therefore  present  in  addition  to  the  infra-red.  There  has  been 
nothing  in  our  experiments  to  indicate  that  the  presence  of  these  visible  rays  modifies  the  results  in  any  way. 


Fig-  73- 

Decay  curves  with  and  without  infra-red. 
X  =0.474  M- 


OF   RED   AND   INFRA-RED   RAYS   UPON   SIDOT   BLENDE.       8 1 


upon  a  straight  line,  and  thus  give  a  check  upon  the  accuracy  of  the  obser- 
vations. 

The  remarkable  reversal  in  the  effect  of  infra-red  in  passing  through  the 
luminescence  spectrum  received  ample  qualitative  confirmation.  With 
the  spectrophotometer  set  for  some  region  in  the  blue  or  violet  the  bright- 
ness of  the  field  increased  noticeably  for  a  few  seconds  when  the  screen  was 
exposed  to  infra-red  during  decay,  even  when  the  exposure  first  began 
several  seconds  after  the  end  of  excitation.  The  same  effect  was  observed 
in  the  case  of  "Kmanations-pulver"  and  Balmain's  paint.  In  the  case  of 
the  latter  substance  the  flash  that  accompanied  exposure  to  the  longer 
waves  developed  more  slowly  and  lasted  longer  than  in  the  case  of  Sidot 
blende. 

Two  interpretations  of  the  results  brought  out  in  Figs.  70  to  75  suggest 
themselves.  Neither,  however,  is  wholly  satisfactory. 


Sec. 


3 

Fig-  74-  Fig.  75. 

Decay  curves  with  and  without  infra-red.     X  =0.464  /jt  (Fig.  74)  and  X  =0.445  M  (Fig.  75). 

In  all  of  these  experiments  the  luminescence  spectrum  consisted  of  two 
bands,  namely,  the  green  band  at  0.5  1  n  and  the  violet  band  at  about  0.45  p. 
It  is  possible  that  the  infra-red  rays  retard  the  decay  of  phosphorescence 
in  the  case  of  the  violet  band  and  accelerate  it  in  the  case  of  the  green  band. 
In  the  curves  of  Fig.  75  we  are  dealing  with  the  violet  band  only;  in  Figs. 
70,  71,  72,  and  74  the  light  is  almost  entirely  from  the  green  band;  but  at 
wave-lengths  0.474  n  (Fig.  73)  and  0.464  /*  (Fig.  74)  the  light  entering  the 
collimator  slit  comes  partly  from  one  of  these  bands  and  partly  from  the 
other.  The  violet  band  apparently  decays  more  rapidly  than  the  green 
band.  (Compare  the  slant  of  the  line  for  /~*  in  Fig.  75  and  Fig.  72.) 
If  the  violet  band  is  initially  the  brighter  of  the  two  the  retarding  effect 
of  the  infra-red  upon  the  decay  of  this  band  will  predominate  in  the  early 
stages  of  decay.  Later,  when  the  violet  band  has  nearly  died  out  and  the 
light  is  chiefly  due  to  the  green  band,  the  opposite  effect  will  predominate. 
The  two  curves  /  and  /'  will  therefore  intersect,  as  shown  in  Fig.  72  and 
Fig-  74- 

Two  objections  may  be  urged  to  this  explanation.  If  the  light  in  the 
case  of  Figs.  73  and  74  is  from  two  bands  that  decay  at  different  rates  we 
should  hardly  expect  the  relation  between  t  and  7~*  to  be  as  simple  as  the 


82  STUDIES  IN  LUMINESCENCE. 

linear  relation  that  holds  for  the  green  band  alone.  Yet  the  deviation  from 
a  linear  relation  in  both  these  cases  is  well  within  the  errors  of  observation. 
Again,  if  the  infra-red  rays  increase  the  brightness  of  the  violet  band  after 
excitation  has  ceased  it  would  seem  reasonable  to  expect  a  similar  effect 
during  excitation.  Yet  the  effect  during  excitation  (Fig.  67)  is  nearly  the 
same  for  both  bands. 

We  were  first  led  to  expect  increased  brightness  in  the  violet  during 
exposure  to  the  infra-red,  and  to  undertake  experiments  in  the  hope  of 
detecting  such  an  effect,  as  the  result  of  an  entirely  different  line  of  reason- 
ing. Looking  upon  phosphorescence  as  due  to  the  recombination  of  ions 
dissociated  by  the  action  of  the  exciting  light,  we  may  explain  the  fact 
that  the  phosphorescence  light  is  of  greater  wave-length  than  the  exciting 
light  (Stokes's  law)  briefly  as  follows:  Dissociation  results  from  the  violent 
resonant  vibration  of  a  neutral  molecule  of  the  active  substance  under  the 
influence  of  the  exciting  waves.  The  wave-length  of  maximum  resonance 
and  therefore  maximum  excitation  is  determined  by  the  natural  period  of 
vibration  of  the  active  molecule,  which  is  influenced  to  some  extent,  but 
not  greatly,  by  the  surrounding  solvent.  The  charged  ions  resulting  from 
excitation  will,  however,  be  attracted  by  the  neutral  molecules  of  the  sol- 
vent and  will  form  the  nuclei  of  heavy  aggregations  of  molecules;  and 
recombinations  of  the  ions  will  therefore  occur  under  conditions  which 
make  the  resulting  vibrations  longer,  on  the  whole,  than  the  period  of  the 
active  molecules  before  dissociation;  hence  the  well-known  displacement 
of  the  luminescence  spectrum  with  reference  to  the  absorption  spectrum. 

Now  the  effect  of  the  infra-red  rays  may  be  to  so  shake  up  the  molecules 
of  the  solvent  as  to  prevent  the  loading  down  of  the  ions  by  the  attraction 
of  neutral  molecules,  or  to  destroy  such  heavy  aggregations  if  already 
formed.  Under  the  influence  of  the  infra-red,  therefore,  the  light  emitted 
will  be  due  largely  to  the  vibrations  that  occur  during  the  recombination 
of  unloaded  ions  and  will  be  of  the  same  wave-length  as  that  which  the 
active  substance  absorbs.  If  the  screen  is  exposed  to  infra-red  rays  after 
excitation  we  should  expect  a  decrease  in  the  intensity  of  phosphorescence 
throughout  the  phosphorescence  band  due  to  the  breaking  down  of  the 
groups  of  molecules  referred  to  above.  But  owing  to  the  resulting  increase 
in  the  number  of  unloaded  ions  we  should  also  expect  the  emission  of  light 
whose  wave-length  is  that  of  the  resonant  absorption  band  of  the  substance. 
Now  the  absorption  band  always  lies  on  the  ultra  side  of  the  luminescence 
band,  and  usually  the  two  bands  overlap.  (That  this  is  the  case  with  Sidot 
blende  is  shown  by  the  fact  that  this  is  one  of  the  substances  for  which 
Stokes's  law,  in  its  strict  form,  is  violated.)  Exposure  to  infra-red  should 
therefore  produce  increased  intensity  near  and  beyond  the  violet  edge  of  the 
phosphorescence  band;  which  is  exactly  what  we  have  observed. 

In  the  region  where  the  absorption  and  emission  spectra  overlap,  the 
effect  will  be  more  complicated.  While  the  light  in  this  region  due  to  the 
ordinary  luminescence  band  will  diminish,  there  will  be  at  the  same  time 
a  temporarily  increased  emission  due  to  the  recombination  of  the  unloaded 
ions  that  are  shaken  loose  by  the  infra-red  vibrations.  A  bright  flash 
immediately  after  the  exposure  to  infra-red,  followed  by  decay  more  rapid 
than  the  normal,  is  therefore  to  be  expected  in  the  intermediate  region 
corresponding  to  Figs.  73  and  74. 


INFLUENCE   OF  RED  AND   INFRA-RED   RAYS   UPON   SIDOT  BLENDE.       83 

The  effect  of  the  longer  waves  during  excitation  is  unfortunately  as  hard 
to  reconcile  with  this  explanation  of  the  phenomena  as  with  that  first 
suggested. 

VARIATIONS   OF  THE   EFFECT   WITH  THE   LENGTH   OF    THE    LONGER  WAVES. 

In  the  case  of  several  phosphorescent  substances,  including  Sidot  blende, 
the  effect  of  rays  of  different  wave-length  in  suppressing  phosphorescence 
has  been  studied  photographically  by  Dahms.1  Our  own  results,  obtained 
by  an  entirely  different  method,  in  general  confirm  his  conclusions  in  a  very 
satisfactory  way. 

The  arrangement  of  apparatus  was  similar  to  that  used  in  our  first  experi- 
ments on  the  decay  of  phosphorescence.2  A  spark  was  used  for  excitation 
and  a  shutter  was  so  arranged  that  the  screen  was  exposed  to  the  infra-red 
rays  and  the  phosphorescent  light  allowed  to  fall  on  the  slit  of  the  spectro- 
photometer  at  the  same  instant  that  the  excitation  was  brought  to  a  close 
by  short-circuiting  the  spark.  A  Nernst  glower  was  used  as  a  source  of 
infra-red  rays.  This  was  mounted  in  the  place  of  the  slit  of  a  large  mirror 
spectrometer  having  a  quartz  prism.3  The  Sidot-blende  screen  was  covered 
with  black  paper  except  for  a  narrow  rectangular  region  having  about  the 
same  width  as  the  Nernst  glower.  The  adjustment  of  the  spectrometer 
having  been  determined  by  observations  in  the  visible  spectrum,  wave- 
lengths in  the  infra-red  were  computed  from  the  angle  of  deviation.  The 
effect  of  different  rays  from  the  Nernst  glower  was  measured  by  the  differ- 
ence between  the  times  required  for  the  phosphorescence  to  fall  from  its 
initial  intensity  to  a  definite  final  intensity  with  and  without  exposure  to 
the  rays  to  be  tested.  Each  point  of  the  curves  shown  in  Fig.  76  is  deter- 
mined from  the  average  of  ten  observations  with  infra-red  and  ten  without, 
the  observations  being  made  alternately.  The  difference  between  the  two, 
expressed  as  a  fraction  of  the  normal  time  of  decay,  has  been  plotted  for 
the  different  wave-lengths  used,  which  ranged  from  0.6/1  to  2.3^.  The 
observations  refer  to  the  region  of  maximum  intensity  in  the  phosphores- 
cence spectrum  (0.5 12  /*)• 

Referring  to  Fig.  76,  it  will  be  seen  that  the  effect  of  the  longer  waves  is 
observable  to  some  extent  in  the  visible  region.  A  maximum  is  reached  at 
about  0.9  fjL,  followed  by  a  minimum  at  about  i.o  n  and  another  maximum 
at  1.3  ju.  From  1.2  /*  on  the  observations  were  repeated  under  slightly 
different  conditions  the  following  day  (see  broken  line).  In  this  case  the 
chief  maximum  appears  to  lie  at  i.37ju.  The  results  are  probably  in  all 
cases  uncertain  to  the  extent  of  2  or  3  per  cent,  and  errors  are  especially 
likely  to  be  serious  in  regions  where  the  effect  is  small.  For  this  reason  we 
can  not  feel  certain  of  the  third  maximum  at  2.18  n,  although  the  probability 
is  that  it  really  exists.  As  far  as  any  important  effect  is  concerned,  how- 
ever, our  results  confirm  the  conclusion  of  Dahms  that  the  action  does  not 
extend  beyond  1.5  IJL. 

It  can  scarcely  be  doubted  that  absorption  of  the  active  rays  is  necessary 
before  they  can  produce  any  effect  upon  phosphorescence.  It  seems 

'Dahms;  Annalen  der  Physik,  13,  p.  425. 
2See  Chapter  III  of  this  memoir. 

'Tests  (by  direct  eye  observation)  with  a  rock-salt  prism  showed  that  no  effect  was  observable  for  wave- 
lengths longer  than  those  transmitted  by  quartz. 


84 


STUDIES  IN  LUMINESCENCE. 


probable,  therefore,  that  Sidot  blende  possesses  broad  absorption  bands 
with  maxima  not  far  from  0.9  fj.  and  1.35/1-  Experiments  with  other  phos- 
phorescent substances  having  ZnS  as  their  base  will  be  necessary  to  deter- 
mine whether  the  absorption  that  determines  this  effect  on  phosphorescence 
is  characteristic  of  the  solvent  (ZnS)  or  of  the  dissolved  metal  causing  lumi- 
nescence. It  is  interesting  to  note,  however,  that  in  the  absorption 


36 
24 
22 

20 
18 
16 

•fJ    14 

(L> 
O   12 

&  10 
8 
6 
4 
2 
0 
-2 


0.7  0 


8    0 


/ 


9    1.0    I 


7 


Z 


7    1.8/1 


Fig.  76. 


Effect  of  infra-red  rays  of  different  wave-length.     Ordinates  represent  the  percentage  diminution  in  the 
time  of  decay  under  the  influence  of  infra-red. 

spectrum  of  sphalerite  (ZnS)  Coblentz  has  found  evidence  of  bands 
at  about  0.9  n  and  1.4/1.  While  Coblentz's  work  does  not  indicate  great 
absorption  at  these  points,  it  must  be  remembered  that  they  fall  in  the 
most  intense  region  of  the  spectrum  of  a  Nernst  glower,  so  that  the  total 
amount  of  energy  absorbed  might  be  very  considerable. 


CHAPTER  VI. 

VARIATIONS  IN  THE  DECAY  OF  PHOSPHORESCENCE  PRODUCED 

BY  HEATING. 

In  connection  with  the  investigations  described  in  Chapters  IV  and  V 
Dr.C.  A.  Pierce,  during  the  years  1906-1908,  made  an  extended  study  of  the 
phosphorescence  of  Sidot  blende  and  of  Balmain's  paint  at  temperatures 
ranging  from  room  temperature  to  300°  C.  The  present  chapter  con  tains  a 
summary  of  his  results.1 

EXPERIMENTS  WITH  SIDOT  BLENDE. 

The  apparatus  used  was  similar  in  principle  to  that  described  in  Chap- 
ter IV,  p.  58,  but  was  modified  to  adapt  it  to  the  conditions  of  the  experi- 
ment. The  substance  studied  was  the  phosphorescent  zinc  sulphide  already 
referred  to  as  "Emanations-pulver."  It  was  placed  in  a  shallow  dish  and 
heated  by  means  of  an  electric  furnace  capable  of  being  readily  removed  and 
replaced  in  its  position  relative  to  the  powder  without  disturbing  the  latter. 

The  temperature  of  the  powder  was  measured  by  means  of  a  thermo- 
junction  embedded  in  the  mass.  Phosphorescence  was  excited  by  a  mer- 
cury-arc lamp  of  the  Lummer  type. 

By  means  of  properly  placed  mirrors  the  exciting  light  was  reflected  upon 
the  surface  of  the  powder  and  the  phosphorescent  light  was  reflected 
through  a  double  window  of  mica  and  plate  glass  into  the  field  of  a  Lummer- 
Brodhun  photometer.  The  adjustments  were  such  that  the  conditions  of 
illumination  and  observation  were  the  same  whether  the  powder  was  within 
the  furnace  or  outside.  The  comparison  light  was  an  acetylene  flame  with 
diaphragm.  A  color  match  was  obtained  by  the  interposition  of  colored 
glasses  and  the  balance  of  illumination  in  the  field  of  the  photometer  by 
moving  the  comparison  light. 

The  apparatus  was  arranged  so  that  a  single  observer,  working  in  a  dark 
room,  could  automatically  record  all  observations  such  as  the  times  of  open- 
ing and  closing  the  shutter  of  the  exciting  lamp,  the  beginning  and  end  of 
heating,  the  times  at  which  photometric  observations  were  made,  and  the 
position  of  the  comparison  light  on  the  photometer  bar.  The  observer 
could  likewise  make  all  manipulations  during  a  complete  run  without  taking 
his  eye  from  the  observing  telescope.  Before  each  set  of  observations 
the  powder  was  exposed  to  infra-red  rays  for  i  minute  and  allowed  to  re- 
main in  darkness  for  5  minutes  before  excitation  in  order,  as  already  de- 
scribed in  Chapter  IV,  to  secure  a  standard  condition.  In  the  present  case 
the  action  of  the  infra-red  was  not  sufficient  to  destroy  all  thermo-lumines- 
cence  when  the  powder  was  heated  to  350°  C.,  but  the  effect  was  so  reduced 
that  even  after  the  strongest  previous  excitation  the  thermo-luminescence 
could  not  be  measured  with  the  photometer.  The  infra-red  rays,  which 

'C.  A.  Pierce,  Physical  Review,  xxvi,  pp.  312  and  454. 

85 


86 


STUDIES   IN   LUMINESCENCE. 


were  from  a  16  c.p.  incandescent  lamp  with  a  screen  of  thin  vulcanite, 
heated  the  powder  slightly,  but  the  wait  of  5  minutes  allowed  it  to  cool 
to  approximately  room  temperature. 

The  effect  of  temperature  upon  the  decay  curve,  when  the  Sidot  blende 
is  excited  and  allowed  to  decay  at  the  same  temperature,  is  shown  in  Fig.  77. 


60 


70 


Fig.  77. 

Effect  of  temperature  during  excitation  and  decay.     Excited  40  seconds.     Curve  A.  temperature  21°  C.; 
curve  B,  temperature  37°  C.;  curve  C,  temperature  60°  C.;  curve  D,  temperature  85°  C. 

The  curves  are  plotted  with  distances  of  the  standard  lamp  from  the  photo- 
meter screen  as  ordinates  and  time  reckoned  from  the  end  of  excitation  as 
abscissas.  The  ordinates  are  therefore  inversely  proportional  to  the  square 
roots  of  the  intensities.  Curves  A  and  B  are  concave  downward  through- 
out, while  C  and  D  are  approximately 
straight  lines.  In  fact,  the  points  on 
curve  D  indicate  an  upward  bending. 
The  initial  intensity  evidently  increases 
and  the  decay  becomes  more  rapid  as 
the  temperature  is  raised. 

To  further  test  the  form  of  the  curve 
of  decay  at  high  temperatures,  a  series 
of  runs  was  made  with  constant  length 
of  excitation  and  constant  temperature. 
Four  runs  of  the  series  are  shown  in 
Fig.  78.  If  the  curves  had  been  plotted 
with  the  actual  times  as  abscissas,  they 
would  coincide.  In  order  that  the  eye 
may  be  able  to  distinguish  readily  the 
points  on  each  curve,  the  four  curves 
are  separated  by  plotting  curve  D  with 
the  abscissas  marked  on  the  figure  and 
displacing  the  remaining  curves  each 
5  seconds  farther  to  the  right.  It  is 

evident  that  the  curves  are  straight  lines  throughout  the  time  of  observa- 
tion, but  it  is  probable  that  they  would  show  a  downward  bending  nearer 
the  origin.  Studies  of  curves  of  very  rapid  decay,  to  be  described  in 
Chapter  VII,  greatly  strengthen  this  view.  The  conclusion  that  at  room 


Fig.  78. 

Effect  of  temperature  during  excitation  and 
decay.  Excited  400  seconds.  Tempera- 
ture 93°  C. 


DECAY  OF  PHOSPHORESCENCE  PRODUCED  BY  HEATING. 


temperature  the  observed  part  of  the  decay  curve  is  concave  downward 
and  becomes  less  concave  as  the  temperature  is  raised  is  substantiated  by 
several  series  of  curves  which  are  not  reproduced. 

A  possible  explanation  of  the  changes  in  the  decay  curve  that  occur 
when  Sidot  blende  is  excited  at  different  temperatures  may  be  deduced  as 
follows :  Let  it  be  assumed  that  the  law  of  decay  for  a  single  band  in  the 
spectrum  is1 

7-      ' 


Let  it  be  assumed  that  the  phosphorescent  spectrum  of  Sidot  blende  con- 
sists of  two  bands,  and  let  curves  AB  and  CD,  Fig.  79,  represents  the  decay 
of  these  bands.  The  decay  of  the  total  intensity  can  be  computed  by  the 
equation2 


and  is  represented  in  Fig.  79  by  the  curve  OP.  This  curve  is  concave 
downward  throughout  but  approaches  a  straight  line  with  increase  in  time. 
From  Figs.  77  and  78  it  can  be  seen  that  the  effect  of  a  higher  temperature 
is  to  hurry  the  decay.  Hence  the  slope  of  the  lines  representing  the  decay 


Fig-  79- 

AB,  decay  of  one  band  at  a  certain  temperature; 
CD,  decay  of  the  other  band  at  the  same  tem- 
perature; OP,  decay  of  the  total  intensity  due  to 
the  two  bands;  EF,  decay  of  the  first  band  at  a. 
higher  temperature;  GH,  decay  of  the  other  band 
at  the  higher  temperature;  MN,  decay  of  the 
total  intensity  at  the  higher  temperature. 


of  the  separate  bands3  will  be  greater  at  a  higher  temperature.  MN,  Fig. 
79,  represents  the  decay  of  the  total  intensity  at  the  higher  temperature. 
M  N  is  concave  downward  throughout,  but  if  the  portion  occurring  within 
the  first  few  seconds  were  not  drawn,  the  remainder  of  the  curve  would 

'The  law  proposed  by  H.  Becquerel.     See  Chapter  XV. 

2The  law  proposed  by  H.  Becquerel  for  the  decay  of  the  total  intensity  when  the  phosphorescent  spectrum 
consists  of  more  than  one  band. 

•The  existence  of  two  bands  in  the  fluorescence  spectrum  of  Sidot  blende  has  been  demonstrated  in 
Chapter  IV.  The  presence  of  two  such  bands  in  the  case  of  the  powder  studied  by  Dr.  Pierce  was  established 
by  him  by  means  of  spectrophotometric  measurements.  Their  maxima  were  found  to  be  approximately 
at  0.53  M  and  0.41  ft.  The  same  bands  might  therefore  be  expected  in  the  phosphorescence  spectrum. 


STUDIES   IN   LUMINESCENCE. 


appear  to  be  a  straight  line,  while  under  the  same  conditions  the  curve  OP 
would  still  be  seen  to  be  concave  downward. 

The  curves  in  Fig.  79  represent  closely  the  facts  deduced  from  Figs.  77 
and  78.  They  also  agree  with  the  observations  recorded  in  Chapter  IV, 
which  show  that  the  decay  curve  consists  of  two  straight  lines  gradually 
merging  into  one  another.  By  changing  the  slopes  and  intercepts  of  the 
lines  in  Fig.  79  representing  the  decay  of  the  single  bands,  a  decay  curve  of 
the  total  intensity  can  be  derived  which  will  exactly  fit  the  deductions 
mentioned  above.  Dr.  Pierce's  later  experiments  on  the  decay  in  Sidot 
blende  (Chapter  VIII)  nevertheless  make  it  seem  very  doubtful  whether 
this  explanation  of  the  form  of  the  decay  curve,  and  of  its  dependence  upon 
temperature,  can  be  made  to  accord  with  all  the  experimental  facts. 

The  effect  of  heating  the  Sidot  blende  after  it  has  been  excited  at  room 
temperature  and  allowed  to  decay  at  room  temperature  to  a  small  fraction 
of  its  initial  intensity  of  phosphorescence  is  illustrated  in  Figs.  80,  8 1 ,  and  82 . 
The  result  of  this  process  is  the  well-known  phenomenon  of  thermo-lumines- 
cence.  The  curves  are  plotted  with  intensities  as  ordinates  and  times,  meas- 
ured from  the  moment  of  heating,  as  abscissas.  Higher  temperatures  increase 
the  intensity  of  the  outburst  of  light,  and  cause  the  maximum  to  occur 
sooner  after  the  beginning  of  heating  and  to  die  away  more  rapidly. 

In  the  case  of  Fig.  80  the  initial  intensity  immediately  after  excitation 
was  somewhat  greater  than  7=  12.  The  intensity  was  allowed  to  decay  to 
I  =  0.8,  when  heating  was  begun.  In  the  case  of  Fig.  82,  the  initial  intensity 
was  somewhat  greater  than  7  =  37- 


\ 


\ 


Fig.  80. 

Effect  of  heating  when  the  phosphorescence  has  decayed  to  an  intensity  equal  to  0.8.  Excited  10 
seconds  at  room  temperature.  Curve  A,  heated  to  307°  C.;  curve  H.  heated  to  270°  C.; 
curve  C,  heated  to  208°  C.;  curve  D,  heated  to  155°  C.;  curve  E,  heated  to  99°  C. 

The  effect  of  heating  after  excitation  may  be  considered  in  one  of  two 
ways.  Either  it  suddenly  releases  the  energy  represented  by  the  phos- 
phorescence, or  else  it  sets  up  some  new  reactions  in  the  powder.  Though 
the  decay  had  reached  a  low  intensity  before  heating  was  begun  in  the  runs 
shown  in  Figs.  80,  8i,and  82,  yet  at  this  low  intensity  the  decay  was  slow; 
hence  there  may  have  been  considerable  energy  still  left,  which  heating 


DECAY   OF   PHOSPHORESCENCE   PRODUCED   BY   HEATING. 


89 


released  to  give  the  flash  peculiar  to  thermo-luminescence.  If  this  is  the 
case,  the  areas  between  the  curves  in  either  Fig.  80,  81,  or  82  and  the  coor- 
dinate axes  should  be  equal  to  each  other.  It  is  impossible  to  get  experi- 
mental data  with  which  to  draw  the  curves  to  the  axis,  so  the  author  pro- 
jected the  curves  tentatively.  This  is  not  an  entirely  rash  thing  to  do, 


Fig.  81. 

Curves  similar  to  those  in  Fig.  81.  Excited  80  seconds  at  room  temperature.  Curve  A,  heated  to 
308°  C.;  curve  B,  heated  to  267°  C.;  curve  C,  heated  to  206°  C.;  curve  D,  heated  to  155.5°  C. ; 
curve  E,  heated  to  99°  C. 

because  a  slight  variation  in  the  prolongations  will  have  effect ;  furthermore, 
while  the  low  intensities  were  not  measurable,  one  could  nevertheless  get 


Z 


£ 


Fig.  82. 

Curves  similar  to  those  in  Figs.  80  and  81.  Excited  320  seconds  at  room  temperature.  Curve  A, 
heated  to  309°  C.;  curve  B.  heated  to  266°  C.;  curve  C.  heated  to  207°  C.;  curve  D.  heated  to 
153°  C.;  curve  F,  heated  to  98°  C. 

some  idea  of  the  rapidity  of  decay  by  noticing  how  rapidly  the  photometer 
screen  became  dark. 

Fig.  83  shows  the  areas  plotted  with  temperatures  as  abscissas. 

If  the  areas  had  been  equal  to  each  other  for  a  given  excitation,  each  of 
the  curves  in  Fig.  83  would  have  been  a  straight  line  parallel  to  the  tempera- 
ture axis.  Aside  from  the  low  values  at  300°,  which  are  probably  due  to 


9o 


STUDIES   IN   LUMINESCENCE. 


the  approach  to  temperatures  at  which  the  substance  loses  its  power  to 
phosphoresce,  the  curves  suggest  by  their  approximation  to  horizontal 
lines  that  the  function  of  heating  is  merely  to  release  the  stored  energy 
more  suddenly. 

If  this  explanation  of  the  effect  of  heating  is  correct,  then  it  would  be 
reasonable  to  expect  that  in  the  case  of  the  curves  of  Fig.  77  the  decay  would 
be  more  rapid  the  higher  the  temperature,  which  effect  has  already  been 
pointed  out. 

Fig.  84  shows  the  change  in  the  maximum  ordinate  of  the  curves  of  thermo- 
luminescence  as  the  temperature  is  raised.  No  weight  is  given  to  the  exact 
shape  of  the  curves,  the  points  being  connected  merely  to  aid  the  eye  in 
distinguishing  them.  The  curves  are  in  accordance  with  what  would  be 
expected  if  the  function  of  the  temperature  is  to  liberate  suddenly  the 
phosphorescent  energy.  If  this  is  the  case,  the  greater  the  temperature 
the  quicker  the  energy  will  be  liberated  and  the  brighter  the  flash.  The 
relation  of  maximum  intensity  to  temperature  is  shown  more  clearly  in 
Fig.  85. 

To  ascertain  the  rapidity  with  which  the  powder  within  the  furnace  was 
heated,  measurements  were  made  for  each  of  the  temperatures  at  which 


Fig.  83. 


Fig.  84 


Fig.  85. 


Fig.  83. — Areas  between  the  thermo-phosphorescence  curves  and  the  coordinate  axes.  Curve  .4,  excited 
320  seconds  (from  Fig.  82) ;  curve  B.  excited  160  seconds;  curve  C,  excited  80  seconds  (from  Fig.  81) ; 
curve  D,  excited  40  seconds;  curve  E,  excited  20  seconds;  curve  F,  excited  10  seconds  (from  Fig.  80). 

Fig.  84. — Change  in  the  maximum  intensity  as  the  temperature  is  raised.  Maximum  intensity  vs.  time  of 
maximum  intensity  measured  from  the  beginning  of  heating.  Curve  A,  excited  320  seconds  (from 
Fig.  82);  curve  B,  excited  160  seconds;  curve  C,  excited  80  seconds  (from  Fig.  81);  curve  D,  excited 
40  seconds;  curve  E,  excited  20  seconds;  curve  F,  excited  10  seconds  (from  Fig.  80).  Each  point  on 
a  curve  is  for  a  given  temperature.  On  each  curve  the  lowest  point  is  for  99°  C.  and  for  the  other  points 
in  consecutive  order  155°,  207°,  267°,  and  308°  C.,  respectively. 

Fig.  85. — Increase  of  maximum  intensity  of  thermo-luminescence  with  increase  of  temperature.  Curve  A, 
excited  320  seconds  (from  Fig.  82);  curve  B,  excited  160  seconds;  curve  C,  excited  80  seconds  (from 
Fig.  81);  curve  D,  excited  40  seconds;  curve  E,  excited  20  seconds;  curve  F,  excited  10  seconds 
(from  Fig.  80). 

curves  of  thermo-luminescence  had  been  obtained,  and  from  these  curves 
were  plotted  showing  the  rise  of  temperature  of  powder  and  the  variation 
in  the  temperature  of  the  furnace.  Fig.  86  gives  the  curves  for  two  of  these 
temperatures.  The  general  conclusion  from  these  measurements  was  that 
the  powder  reached  a  constant  temperature  in  a  constant  time  independent 
of  the  temperature.  The  periods  of  the  two  galvanometers  used  in  the 
temperature  measurements  were  too  short  to  affect  the  shape  of  the  curves 
appreciably. 

The  method  of  heating  employed  possesses  several  advantages.  The 
temperatures  are  known  and  can  be  accurately  reproduced  as  many  times  as 
desirable.  Further,  the  gradual  heating  allows  the  flash  to  be  followed  in 


DECAY  OF  PHOSPHORESCENCE;  PRODUCED  BY  HEATING. 


the  photometer.  For  some  other  work  a  strip  of  platinum  which  supported 
a  thin  layer  of  the  powder  was  heated  by  passing  an  electric  current  through 
the  strip.  The  flash  in  this  case  occurred  too  rapidly  to  be  followed,  the 
maximum  intensity  being  the  only  measurement  possible. 


Fig.  86. 

Curves  A  and  C,  variation  in  temperature  of  furnace  when 
the  powder  is  put  in.  Curves  B  and  D.  increase  in  tem- 
perature of  the  powder.  Curves  A  and  B  do  not  become 
identical  because  they  were  not  taken  simultaneously  and 
the  temperature  changed  slightly  between  the  two  runs. 


It  is  not  easy  to  estimate  the  effect  of  gradual  heating  as  compared  with 
more  nearly  instantaneous  heating.  The  outside  layer  of  the  powder  is 
subjected  to  the  temperature  of  the  furnace,  which  does  not  vary  widely. 
It  is  only  the  inside  layers  which  are  heated  as  slowly,  as  the  curves  in  Fig. 
86  indicate.  It  is  not  believed  that  any  material  change  is  introduced  in 


\ 


Fig.  87. 

Effect  of  varying  the  length  of  excitation.  Ordinates  show  the  intensities  of  luminescence,  and  abscissas 
the  times  after  heating  began.  The  length  of  excitation  and  the  temperature  of  the  furnace  are  as 
follows: 

Curve  6,  320  sec.,  153.0°  C.;  curve  5,  160.0  sec.,  155.5°  C.;  curve  4,  80.1  sec.,  155.5°  C.:  curves, 
40.0  sec..  155.5°  C.;  curve  2,  19.9  sec.,  155.5°  C.;  curve  i,  10.0  sec.,  155.0°  C. 

the  relation  of  the  various  curves,  though  the  actual  form  of  the  curves 
may  be  changed  more  or  less. 

The  effect  of  varying  the  length  of  excitation  is  brought  out  in  Figs.  87 
and  88.     As  in  previous  curves,  the  phosphorescence  was  excited  at  room 


92 


STUDIES  IN  LUMINESCENCE. 


temperature  and  allowed  to  decay  to  7  =  o.8  before  heating.  The  points 
are  joined  by  straight  lines  to  aid  the  eye  in  following  the  individual  curves. 
Two  effects  are  noticeable  at  a  glance.  The  maximum  intensity  increases 
with  excitation  and  is  shifted  to  the  right,  *'.  e.,  comes  at  a  later  time.  The 
effect  of  saturation  is  shown.  This  is  brought  out  more  clearly  in  Fig.  89. 


V 


A 


Fig.  88. — Curves  similar  to  Fig.  87.  Effect  of  varying  the 
length  of  excitation.  The  length  of  excitation  and  the 
temperature  of  the  furnace  are  as  follows:  Run  6,  320.1 
sec.,  309°  C.;  run  5.  159.9  sec.,  308°  C.;  run  4,  80.2  sec.. 
308°  C.;  run  3,  39.9  sec.,  308°  C.;  run  2,  20.0  sec.,  309° 
C.;  run  i.  10.0  sec.,  307°  C. 

Fig.  89. — Saturation  effect.  /u  rs.  length  of  excitation. 
Curve  A ,  temperature  308°  C.;  curve  B,  temperature  267° 
C.;  curve  C,  temperature  207°  C.;  curve  D,  temperature 
155.5  C.;  curve  E,  temperature  98°  C. 


Fig,  88.  Fig.  89. 

To  get  the  maximum  point  in  each  run,  smooth  curves,  not  shown  in 
the  figures,  were  drawn  through  the  different  points.  These  curves  show 
that  increasing  the  excitation  beyond  a  certain  length  does  not  increase 
the  energy  manifested  as  thermo-luminescence.  That  saturation  takes 
place  is  further  shown  by  the  time  that  luminescence  lasts.  Figs.  87  and  88 
show  that  the  duration  of  luminescence  has  about  reached  a  maximum. 
The  areas  included  between  the  curves  and  the  coordinate  axes  also  show 


Sec 


Fig.  90. 

Effect  of  delay  in  heating.  Time  measured  from  the  end  of  excitation.  Excited  320  seconds  at  room 
temperature.  Temperature  of  furnace  303°  C.  Curve  A,  decay  at  room  temperature.  The  time 
between  the  end  of  excitation  and  the  beginning  of  heating  is  as  follows: 

Curve  B,  2.5  seconds;  C,  21.4  seconds;  D,  41.3  seconds;  E,  81.1  seconds;  F,  162.0  seconds;  G,  321.6 
seconds;  H,  631.8  seconds;  /,  1280.0  seconds. 

that  further  increase  with  increased  excitation  is  limited.  The  shifting 
of  maximum  intensity  resembles  an  inertia  effect;  the  longer  the  excitation 
the  longer  it  takes  the  temperature  to  produce  the  maximum  thermo-effect. 


DECAY  OF  PHOSPHORESCENCE  PRODUCED  BY  HEATING. 


93 


The  effect  of  delay  between  the  end  of  excitation  and  the  beginning  of 
heating  was  investigated  at  some  length.  The  general  character  of  the 
results  obtained  is  given  in  Fig.  90  and  the  subsequent  diagrams. 

Fig.  90  shows  the  way  in  which  the  outburst  of  thermo-luminescence 
diminishes  in  intensity,  in  the  case  of  a  substance  excited  for  a  given  time 
and  subsequently  heated  in  a  furnace  of  given  temperature,  as  the  interval 
of  time  before  the  beginning  of  heating  is  increased.1  Figs.  92  and  93,  in 
which  time  is  measured  from  the  beginning  of  heating,  indicate  clearly 
a  shift  in  the  time  of  reaching  the  maximum  of  intensity  of  thermo- 
luminescence  in  the  same  direction  as  that  already  noted  in  the  case 
of  increased  length  of  excitation.  The  energy  of  phosphorescence  does 
not  manifest  itself  so  rapidly  after 
long  delay  in  heating  or  long  excita- 
tion as  after  short  delay  or  short  ex- 
citation. For  any  given  excitation 
and  temperature  the  curves  tend  to 
coincide  after  a  given  time,  which 
may  be  taken  to  indicate  that  the 
interval  of  time  occupied  by  an  out- 
burst  of  thermo-luminescence  is  in- 
dependent of  the  time  which  has 
elapsed  since  excitation. 

Table  16  gives  the  approximate 
duration  of  the  flash  of  thermo- 
luminescence  from  nine  sets  of 
observations  under  varying  condi- 
tions  of  excitation  and  heating. 

While  the  times  stated  are  neces- 

sarily   somewhat   inaccurate,    they  indicate  that  brief  outbursts  follow 
short  exposures  and  high  temperatures  of  the  furnace  and  vice  versa. 


A 


*oo 


eoo 


1200  Sec. 


Fig.  gi.1 


Fig.  92. 

Effect  of  delay  in  heating.  Time 
measured  from  beginning  of 
heating.  Excited  320  seconds 
at  room  temperature.  Tem- 
perature of  furnace  93°  C. 
The  time  between  the  end  of 
excitation  and  the  beginning 
of  heating  is  as  follows: 

Curve  i,  a.o  sec.;  curve  2,  21.9 
sec.;  curve  3,  41.5  sec.;  curve 
4,8i.5sec.;  curves,  162. osec.; 
curve  6,  332.0  sec. 


lThe  location  of  the  crests  C,  D,  E,  F,  G,  H.  I  (Fig.  90)  is  along  a  curve  which  suggests  in  its  form  the 
ordinary  curve  of  phosphorescence,  and  when  we  apply  the  usual  criterion,  i.e.,  plotting  /— X  and  times,  we 
get  the  curve  shown  in  Fig.  9 1 ,  which  has  the  significant  form  already  discussed  in  Chapter  IV  of  this  treatise. 


94 


STUDIES  IN  LUMINESCENCE. 


Fig.  94  gives  in  graphical  form  a  summary  of  the  results  of  nine  sets  of 
observations  upon  the  decrease  in  maximum  intensity  of  thermo-lumines- 
cence  with  delay  in  heating.  The  curve  G  is  from  data  plotted  in  Fig.  90. 
In  Fig.  95,  which  is  from  the  same  set  of  observations,  the  time  of  maximum 
intensity  is  measured  from  the  beginning  of  heating  in  order  to  exhibit 
more  clearly  the  shift  of  the  maximum  as  the  result  of  delay  in  beginning 
heating. 


Fig.  93- 

Curves  similar  to  Fig.  93.  Excited  40  seconds  at 
room  temperature.  Temperature  of  furnace  94° 
C.  The  time  between  the  end  of  excitation  and 
the  beginning  of  heating  is  as  follows: 

Curve  A,  1.9  sec.;  curve  B,  22.1  sec.,  curve  C,  42.1 
sec.;  curve  D,  82.6  sec.;  curve  E.  162.1  sec. 


TABLE  16. 

Time  of      Temp,  of      Time  required  for  in- 
excitation.     furnace.         tensity  to  reach  /. 


sec.  °C. 

10  94 

10  200 

10  302 

40  92 

40  2OO 

40  3OO 

320  93 

320  202 

320  303 


sec. 

/=! 

.25          60 

45 

32 

1  =  2 

5        100 

80 

37 

120 

75 

40 

In  Fig.  96  is  shown  the  maximum  intensity  plotted  against  length  of 
excitation.  These  curves  are  similar  to  those  shown  in  Fig.  89,  but  cover 
a  larger  range  of  conditions.  They  indicate  the  same  tendency  to  satu- 


X 


t 


Fig.  94- 

Decrease  in  maximum  intensity 
with  delay  in  heating.  The  co- 
ordinates are  /M  and  time  of  /« 
measured  from  the  end  of  excita- 
tion. The  length  of  excitation 
and  the  temperature  of  the  fur- 
nace are  as  follows: 

Curve  A,  320  sec.     93°  C. 


40  sec. 
10  sec. 


B, 

C, 

D,  320  sec. 

E,  40  sec. 

F,  10  sec. 

G,  320  sec. 
//,    40  sec. 
/,     10  sec. 


93 

92°  C. 
94°  C. 

202°  C. 

200°  C. 

200°  C. 

303°  c. 


302°  C. 


ration  with  increasing  excitation  whatever  be  the  temperature  and  the 
delay  of  heating;  a  point  which  would  undoubtedly  have  been  better 
shown  had  a  greater  number  of  lengths  of  excitation  been  investigated. 

Fig.  97  shows  the  relation  between  maximum  intensity  and  temperature 
for  the  nine  sets  of  runs.  These  curves  correspond  to  those  shown  in  Fig. 
84.  In  some  of  the  cases  but  one  point  can  be  given,  the  other  runs  show- 


DECAY   OF   PHOSPHORESCENCE   PRODUCED   BY   HEATING. 


95 


ing  no  measurable  intensity  or  else  no  maximum.  The  relation  between 
the  maximum  intensity  and  the  temperature,  beyond  the  fact  that  the 
intensity  increases  with  the  temperature,  is  not  clear. 


V 


Fig-  95- 

Maximum  intensity  vs.  time  of  maximum  intensity  measured  from  the  beginning  of  heating. 
For  description  of  curves  see  Fig.  94. 


Fig.  96. 

Maximum  intensity  vs.  length  of  excitation.     The  temperature  of  the  furnace  and  the  time  between  the 

end  of  excitation  and  the  beginning  of  heating  are  as  follows: 
Curve  A,  302°  C.,      21. 6  sec.  Curve  H,  200°  C.,    21. 9  sec.  Curve  M,  93°  C.,    21. 5  sec. 


1,                 41.7                              I,     "         41.. 
81.4                               J,     "          8i.e 

>,                     161.6                                       K,                  161.! 
321-0                                       L,      "          321.' 
636.0 
II55-0 

••>                                        N,      "          41.8 
O,      "          82.0 
J                                        P,                162.0 
'                                Q,             322.0 

Curve  A,  320  sec.,      2i.7sec. 

r'                     t1-3 
C,                       81.3 

D,                    162.0 
E,                    321.8 
^,                     631.8 
G,                    1280.0 
//,4o    sec.,       22.1  sec. 
/,                       41-8 
J,                       81.6 
K,                    161.9 
L,                     321.9 
M  ,                    642  .  o 
N,                   1230.0 
O,  10    sec.,       2t  .8  sec. 
P,                       41-7 
Q,                        81.7 
R,                   161.4 
•S,                    319-5 

I 

i 

.A 

I 

I        ; 

fcO 

n 

>> 

bO 

bO 

n\ 

.« 

40 

/ 

n 

to 

H 

40 

// 

/ 

// 

1 

, 

n 

2 

'i> 

20 

/ 

7 

zo 

// 

f 

,c 

z 

"~? 

J 

^ 

i  O 

0        ' 

// 

jocf 

>o 

}oo-c 

0        ' 

^ 

^^^ 

'iccfc 

o      ' 

loo* 

^=^ 

200' 

El 

Vcrf( 

Fig-  97- 

Maximum  intensity  vs.  temperature  of  furnace.     The  length  of  excitation  and  time  between  end  of 
excitation  and  beginning  of  heating  are  as  shown  herewith. 

In  an  earlier  paragraph  mention  was  made  of  the  existence  of  two  bands  in 
the  phosphorescence  spectrum  of  the  sample  of  Sidot  blende  used  in  this  study. 


96  STUDIES  IN  LUMINESCENCE;. 

If  there  are  two  bands,  it  is  natural  to  expect  some  indication  of  their  exist- 
ence in  the  curves  of  thermo -luminescence.  Referring  to  Fig.  92,  there  will 
be  found  evidence  of  two  bands.  After  the  intensity  has  reached  a  maximum, 
it  falls  off  rapidly  at  first,  then  there  is  an  indication  of  a  slowing  up  in  the 
decay  followed  by  relatively  rapid  decay.  Whether  there  is  a  complete 
bending  back  in  any  of  the  curves  can  not  be  stated,  because  the  points  are 
too  far  apart.  Whether  a  complete  bending  back  is  possible  depends  on  the 
relative  intensities  of  luminescence  of  the  two  bands  and  the  relative  rapidity 
of  decay.  It  has  not  seemed  advisable  to  search  for  more  certain  indications 
of  two  bands  in  the  thermo-phosphorescence  because  of  the  difficulty  of 
increasing  the  number  of  observations  taken  in  a  given  time.  In  part  II 
of  this  chapter,  curves  will  be  shown  in  the  case  of  Balmain's  paint,  which 
decays  slowly  enough  to  be  more  completely  studied.  This  substance  has 
two  bands  in  its  phosphorescence  spectrum,  and  the  curves  of  thermo- 
luminescence  show  two  maxima  under  certain  conditions  of  excitation, 
temperature,  and  delay  in  heating. 

EXPERIMENTS  WITH  BALMAIN'S  PAINT. 

In  the  experiments  with  Balmain's  paint  the  apparatus  used  was  the  same 
as  that  used  in  the  study  of  Sidot  blende.  A  mercury  lamp  was  used  to 
excite  the  calcium  sulphide,  which  was  in  powder  form.  Since  the  lamp  was 
made  of  ordinary  glass  and  the  light  from  it  was  reflected  at  a  mirror  before 
reaching  the  powder,  the  excitation  was  chiefly  due  to  the  visible  spectrum 
in  the  mercury  arc.  Before  each  excitation  the  powder  was  exposed  to 
infra-red  rays  in  an  attempt  to  bring  it  to  a  standard  condition. 

The  action  of  infra-red  rays  upon  the  phosphorescence  of  calcium  sulphide 
is  not  so  strong  as  upon  Sidot  blende.  In  the  latter  case,  as  has  already  been 
shown  in  Chapter  V,  phosphorescence  due  to  a  very  long  excitation  can  be 
destroyed  almost  immediately  by  infra-red  rays,  and  only  the  highest  allow- 
able temperature,  just  under  dull  red  heat,  is  able  to  produce  thermo- 
luminescence  without  renewed  excitation.  In  the  case  of  calcium  sulphide 
a  very  long  exposure  to  infra-red  rays  was  necessary  to  destroy  the  phos- 
phorescence, and  no  exposure  was  found  to  be  long  enough  to  suppress  the 
thermo-luminescence  completely.  At  the  beginning  of  the  experiments 
on  calcium  sulphide  this  fact  was  not  recognized.  If  it  had  been,  the 
powder  could  have  been  brought  to  an  approximately  standard  condition 
by  heating  to  a  temperature  a  little  higher  than  the  highest  temperature 
at  which  thermo-luminescence  was  to  be  studied.  Fortunately,  in  every 
case  the  powder  was  exposed  for  one  minute  to  infra-red  rays  of  constant 
strength,  hence  it  was  always  brought  to  a  semi-standard  condition.  No 
extended  attempt  was  made  to  compare  the  two  methods,  since  the  blue 
phosphorescence  of  calcium  sulphide  is  a  difficult  color  to  measure  in  the 
photometer. 

The  fluorescence  spectrum  of  this  sample  of  calcium  sulphide  is  shown 
in  Fig.  98.  This  curve  was  obtained  by  comparison  with  the  light  reflected 
from  the  surface  of  a  block  of  magnesium  carbonate  illuminated  by  an 
acetylene  flame.  Two  bands  are  indicated  by  the  curve,  one  with  a  maxi- 
mum at  about  0.41  fj.  and  the  other  with  a  maximum  at  about  0.54^. 


DECAY   OF   PHOSPHORESCENCE   PRODUCED   BY   HEATING. 


97 


The  dotted  vertical  lines  show  the  wave-lengths  of  the  lines  of  the  mercury 
arc.  The  figure  indicates  the  presence  of  stray  light  in  the  spectro-photom- 
eter,  which  lowers  the  zero  line  with  respect  to  the  curve  and  unduly 
exaggerates  the  ratio  of  the  maximum  of  the  band  at  0.41  /*  to  the  maximum 
of  the  bandato.54)U.  The 
band  in  the  blue  is,  how- 
ever, evidently  much  more 
intense  than  the  other. 
This  fact  was  shown  by 
the  color  of  the  initial  phos- 
phorescence, which  was 
blue. 

The  effect  upon  the  de- 
cay curve  of  varying  the 
length  of  excitation  is 
shown  in  Fig.  99.  These 
curves  are  plotted  with 
distances  of  the  standard 
light  from  the  photometer 
as  ordinates  and  time 
measured  from  the  end 

of  excitation  as  abscissas.  For  short  excitations  the  curves  are  concave 
downward  throughout,  but  for  longer  excitations  the  bending  is  concen- 
trated in  the  first  part  of  the  curves.  These  curves  are  similar  to  those 
obtained  with  Sidot  blende,  except  that  the  phosphorescence  lasts  much 
longer.  Hence  it  is  possible  to  get  points  relatively  nearer  the  origin.  This 


. 

Fig.  98. 

Fluorescence  spectrum  of  calcium  sulphide  excited  by  the  visible 
spectrum  of  a  mercury  arc. 


O         40        8O       120       /60.       2.00      240       280      320       360  Sec. 

Fig.  99. 

Effect  of  varying  the  length  of  excitation.  Excitation  and  decay  at  room  temperature. 
Curve  .4,  excited  14.4  sec.  Curve  E,  excited    59. 9  sec. 

B,  30.1  F,  150.3 

C,  30 .  i  G,  300 .  o 

D,  60.2  H,        "       600.0 

Curves  .4,  B,  D,  and  F  were  taken  on  the  second  day  and  the  remainder  on  the  first  day  of  the  run. 

fact  allows  one  to  make  a  tentative  deduction  regarding  initial  intensity 
and  length  of  excitation  at  room  temperature.  A  study  of  Fig.  99  indicates 
strongly  that  the  initial  intensity  is  greater  the  longer  the  excitation,  which 
is  the  impression  obtained  when  gettingthe  curves.  Though  it  isimpossible, 


98 


STUDIES   IN   LUMINESCENCE. 


with  this  apparatus,  to  get  a  measurement  much  nearer  the  origin  than 
0.8  of  a  second,  yet  the  eye  is  sensitive  to  part  of  the  change  of  intensity 
before  this,  giving  one  a  means  of  estimating  roughly  the  amount  of  change 
before  the  first  measured  point.  The  saturation  effect  is  prominent,  as 
was  the  case  with  Sidot  blende.  Saturation  is  shown  both  by  the  change 
of  initial  intensity  and  by  the  change  in  slope  of  the  curves  as  the  excitation 
is  increased. 

In  Fig.  99  some  of  the  curves  were  obtained  one  day  and  the  remainder 
on  another  day.  One  would  expect  curves  B  and  C  to  coincide.  The 
difference  between  them  is  probably  due  to  the  fact  that  the  infra-red 
exposure  does  not  reduce  the  powder  to  a  standard  condition.  Curves  D 
and  E  agree  more  closely  than  B  and  C.  A  curve,  not  shown  in  the  figure, 
excited  for  300  seconds,  coincides  with  G.  This  coincidence  may  have 


20 


60  80 

Seconds 

Fig.  too. 


Decay  curves  at  different  temperatures.     Excited  2  minutes. 
Curve  i,  temp,  of  excitation  and  decay,  room  temp. 

2.  "         "         66°  C. 

3.  102° 

4,  145° 

5,  180° 

been  due  to  chance,  but  it  is  what  would  be  expected,  because  the  effect 
of  previous  history  becomes  of  less  importance  as  the  length  of  excitation 
is  increased. 

That  the  lack  of  agreement  of  curves  B  and  C  is  due  to  the  previous 
history  of  the  powder  is  substantiated  by  the  fact  that  curve  A  was  observed 
immediately  before  curve  B,  and  curve  G  immediately  before  curve  C.  No 
deductions  can  be  made  for  curves  D  and  E,  because  the  history  of  the 
powder  previous  to  the  excitation  for  curve  E  is  not  known.  Curve  D 
followed  B  immediately,  but  some  preliminary  work  without  infra-red  was 
done  before  the  infra-red  treatment  preceding  curve  E. 

The  changes  produced  in  the  decay  curve  by  varying  the  temperature  at 
which  excitation  and  decay  take  place  are  shown  in  Fig.  100.  The  part 
that  previous  history  plays  in  these  curves  can  not  be  estimated  accurately, 


DECAY  OF  PHOSPHORESCENCE;  PRODUCED  BY  HEATING. 


99 


but  its  effect  is  small,  because  the  excitation,  which  is  relatively  a  long  one, 
was  not  changed  during  a  set  of  curves.  The  curves  were  taken  in  the 
order  that  they  are  numbered.  Curve  i ,  taken  at  room  temperature,  is 
similar  to  the  typical  decay  curves  shown  in  Fig.  99.  As  the  temperature 
is  raised  the  curves  pass  through  a  series  of  changes.  Curve  2  begins  con- 
cave upward,  but  changes  during  the  decay  to  concave  downward.  Curve 
3  is  practically  a  straight  line  throughout.  As  the  temperature  is  raised 
still  farther,  the  curves  again  become  concave  downward  throughout, 
differing  not  widely  from  the  typical  decay  curves  at  room  temperature. 
Another  set  of  curves  similar  to  those  in  Fig.  100  is  shown  in  Fig.  101, 
where  the  length  of  excitation  is  10  minutes.  The  double  bending1  is  again 
exhibited  in  curve  2. 


I2O  160  200 

Seconds 
Fig.  101. 

Decay  curves  at  different  temperatures.     Excited  10  minutes. 
Curve  i,  temp,  of  excitation  and  decay,  room  temp. 

2,  "        42°    C. 

3,  74° 

4,  101° 

5,  124° 

The  effect  of  previous  history  is  shown  in  Figs.  102  and  103.  These 
curves  might  readily  be  mistaken  for  curves  showing  the  effect  of  varying 
the  length  of  excitation.  Fig.  103  also  shows  the  effect  of  previous  history, 
modified,  however,  with  the  effect  of  temperature. 

The  decay  of  phosphorescence  in  the  case  of  Balmain's  paint  was  studied 
some  years  ago  by  F.  J.  Micheli,2  who  has  published  decay  curves  of  several 
phosphorescent  substances,  among  them  Balmain's  paint.  For  the  sake  of 
comparison,  several  of  these  curves  have  been  replotted  in  Figs.  105  and  106 
with  i/-^I  and  time  as  coordinates.  Fig.  105  shows  the  effect  of  varying  the 
length  of  excitation.  Fig.  106  shows  the  effect  of  varying  the  tempera- 

'When  taking  the  runs  on  which  Fig.  99  is  based,  no  importance  was  given  to  the  single  curve  which  shows 
double  bending.  Later,  the  runs  on  which  Fig.  100  is  based  were  made  and  the  apparatus  was  rearranged 
for  study  of  another  substance  before  the  significance  of  the  double  bending  was  recognized.  On  looking 
over  all  the  curves  taken  on  Balmain's  paint,  other  indications  of  double  bending  were  found,  but  none  of 
them  so  pronounced  as  those  shown  above. 

'-Arch,  des  Sci.  Phys.  et  Nat.,  12,  p.  5,  1901. 


IOO 


STUDIES   IN   LUMINESCENCE. 


ture  at  which  excitation  and  decay  take  place.  These  curves  and  the 
curves  given  earlier  in  this  article  agree  generally.  Fig.  106  shows  the 
effect  of  excitation  and  decay  at  a  temperature  lower  than  any  used  in  this 
article. 


e 


B 


40 


60 


80 
Fig.  i 02. 


100 


MO     Sec. 


Effect  of  history  previous  to  excitation.  Excitation  and  decay  at  room  temperature.  Excited  30  seconds. 
Curve  A,  excited  after  action  of  infra-red-  curve  B,  excited  immediately  after  curve  A;  curve  C, 
excited  immediately  after  curve  B;  curve  D,  before  exciting  for  curve  D,  the  powder  was  excited  for  10 
minutes  and  allowed  to  decay  to  an  intensity  corresponding  to  D  =9  in  the  figure,  at  which  time.the 
excitation  of  30  seconds  was  begun. 

In  the  case  of  calcium  sulphide,  it  is  not  necessary  to  use  a  spectrophoto- 
meter  to  prove  that  there  is  more  than  one  band  included  in  the  phenom- 
enon of  phosphorescence,  for  under  the  influence  of  heat  the  color  of  the 
powder  can  be  seen  to  change  from  blue  to  green. 


6\0 


&O 


IQQ_  Bee 


Fig.  103. 

Curves  of  Fig.  102  repeated  at  a  temperature  of  133°  C. 

It  has  already  been  shown  that  the  form  of  the  phosphorescence  decay 
curve  may  be  closely  approximated  on  the  basis  of  two  bands,  each  follow- 
ing the  law 

i 


This  explanation  was  carried  to  some  length  in  the  earlier  part  of  this 
chapter.  But  there  seems  to  be  no  way  of  combining  two  decays,  each  of 
which  follows  this  law  so  as  to  produce  an  upward  bending.  If  it  be  sup- 
posed that  one  band  increases  in  intensity  as  the  other  decreases,  then  an 


DECAY   OF  PHOSPHORESCENCE   PRODUCED   BY  HEATIXG. 


IOI 


upward  bending  is  possible ;  but  a  downward  bending  is  impossible  unless 
both  the  bands  subsequently  decay  together.  This  suggests  that  one  of 
the  bands  may  be  due  to  some  secondary  effect,  instead  of  being  produced 


0  20  40  60  80  100  1 2O  140 

Seconds 
Fig.  104. 

Effect  of  history  previous  to  excitation.  Excited  30  seconds.  Before  each  excitation  the  powder  wa3 
excited  for  10  minutes  and  allowed  to  decay  to  an  intensity  corresponding  to  D  =9,  at  which  time  the 
excitation  of  30  seconds  was  begun. 

Curve  C,  excitation  and  decay  at  room  temp. 

B,          "  88°  C. 

A.        "  "        "        "  133° 


8  10 

Minutes 

Fig.  105. 

Effect  of  varying  the  length  of  excitation.     Excitation  and  decay  at  a  temperature  of  18°  C.  (replotted  from 

data  of  Micheli). 

Curve  A,  excited  5  seconds;  B,  20  seconds;  C,  300  seconds.     Another  curve  excited  for  60  seconds  coin- 
cided almost  exactly  with  curve  C. 


Fig.  106. 

Effect  of   varying   the  temperature  at  which    excitation   and  decay  take  place.     Excited  300  seconds 

(replotted  from  data  of  Micheli). 
Curve  A,  temp,  of  excitation  and  decay,  — 21°  C. 

B,  "        "  "  "         "         100° 

C,  18° 

by  the  exciting  light.  Such  a  band  would  increase  in  intensity  a  certain 
length  of  time,  then  decrease  in  intensity.  If  we  suppose  that  the  first 
band  follows  the  law  suggested,  and  that  the  second  band  follows  the  same 


102  STUDY   IN   LUMINESCENCE. 

law  after  it  has  reached  its  maximum  intensity,  then  the  discussion  pre- 
viousty  given  still  holds  good,  the  supposition  being  that  the  second  band 
is  completely  formed  before  the  first  point  on  a  curve  can  be  measured.  If, 
however,  conditions  can  be  changed  so  that  a  point  can  be  observed  before 
the  second  band  is  completely  formed,  then  the  curve  will  show  initially 
an  upward  bending,  as  in  curve  2,  Figs.  99  and  100.  If  a  band  is  formed 
by  some  secondary  effect  it  will  usually  be  present  in  the  fluorescence 
spectrum  because  the  excitation  is  continued  long  enough  for  the  band  to 
be  formed. 

The  effect  of  previous  history  can  be  explained  without  adding  any  com- 
plications to  the  ideas  already  set  forth.  In  the  case  of  Sidot  blende,  it 
was  undoubtedly  the  band  of  longer  wave-length  that  decayed  more  slowly. 
The  same  is  true,  probably,  for  calcium  sulphide,  for  the  color  changes 
from  blue  to  green  when  the  powder  is  heated.  In  the  case  of  the  decay 
curve  at  room  temperature,  the  intensity  is  too  small  for  the  change  in 
color  to  be  recognized  by  the  eye.  Assuming  that  the  band  of  longer 
wrave-lengths  decays  more  slowly,  and  that  this  band  is  due  to  some 
secondary  effect,  then  the  action  of  repeated  excitations  without  inter- 
vening treatment  with  infra-red  might  be  to  increase  the  intensity  and  to 
decrease  the  rapidity  of  decay  of  the  second  band  without  introducing  any 
change  in  the  rapidity  of  decay  of  the  first  band.  Some  indications  of 
these  effects  can  be  seen  in  Figs.  101  and  102. 

It  is  unfortunate  that  the  curves  in  Figs.  101  and  102  were  not  taken 
in  the  reverse  order,  i.  e.,  excitation  of  long  duration  followed  by  shorter 
excitations.  Such  curves,  according  to  the  ideas  just  set  forth,  should 
show  parallelism  after  the  decay  had  proceeded  some  time.  Several  of  the 
sets  of  curves  described  in  Chapter  IV  were  taken  in  this  order  and  they 
exhibit  parallelism  after  decaying  a  short  time.  To  explain  completely 
the  curves  in  Figs.  99  and  100  on  the  basis  of  a  second  band  due  to  secondary 
causes  is  impracticable,  because  the  problem  is  complicated  by  the  addition 
of  the  effect  of  temperature.  One  assumption  will  necessarily  have  to  be 
added,  i.  e.,  that  there  are  certain  temperatures  at  which  either  one  or  the 
other  of  the  bands  is  most  readily  formed.  This  is  a  possible  explanation 
of  the  fact  that  the  decay  curve  becomes  a  straight  line  at  one  temperature. 

A  number  of  sets  of  observations  were  made  to  show  the  effect  of  heating 
after  decay  has  begun.  In  these  experiments  the  powder  was  exposed  to 
infra-red  rays,  then  excited  and  allowed  to  decay  at  room  temperature,  heat- 
ing being  begun  at  a  definite  point  on  the  decay  curve.  Figs.  107  and  108, 
in  which  the  coordinates  are  intensity  and  time,  show  the  effect  of  varying 
the  length  of  excitation.  The  double  flash  mentioned  in  the  case  of  Sidot 
blende  is  plainly  evident  in  the  curves,  and  in  the  experiment  it  was  evident 
to  the  unaided  eye.  The  color  also  varied,  being  blue  for  the  first  flash  and 
a  yellowish  green  for  the  second.  At  the  longest  excitation  the  second 
flash  is  barely  visible  in  the  photometer,  the  only  evidence  of  its  existence 
being  a  slowing  up  in  the  rate  of  decay.  As  the  length  of  excitation  is 
decreased,  the  second  flash  becomes,  relative  to  the  first,  greater  and  greater. 

At  lower  temperatures  the  second  flash  becomes  less  evident,  and  this 
peculiarity  of  the  curve  for  600  seconds  excitation,  as  shown  in  Figs.  107 
and  1 08,  no  longer  exists.  For  the  lower  temperatures  the  second  flash  for 


DECAY   OF   PHOSPHORESCENCE    PRODUCED   BY   HEATING. 


103 


600  seconds  excitation  is  greater  than  that  for  any  shorter  excitation  at  the 
same  temperature.  At  a  temperature  near  100°  C.,  the  second  flash  is  no 
longer  evident  in  the  curves. 


160  200          £40 

Fig.  107. 

Effect  of  varying  the  length  of  excitation.     Excited  and  allowed  to  decay  at  room  temperature  to  I  =2.9, 

when  heating  was  begun.     Temperature  of  furnace,  277°  C. 
Curve  A,  excited  600.2  sec.  Curve  D,  excited  60.7  sec. 

B,  300.0  E,          "       30.1 

C.  150.1  F,          "       14.8 


eo        joo 

Fig.  108. 

Curves  similar  to  Fig.  107.  Temperature  of  furnace,  339°  C. 
Curve  i ,  excited    600.0  sec.  Curve  4,    excited  59.8  sec. 

2,  300.0  5,  29.8 

3,  150.1  6,  14.2 


wo    i     Sec. 


STUDIES   IN   LUMINESCENCE. 


It  is  evident  that  two  bands  are  represented  in  the  preceding  curves. 
Any  one  of  these  curves  is  probably  made  up  by  the  superposition  of  two 
curves,  each  of  these  component  curves  representing  the  flash  for  one  of  the 
bands.  If  one  considers  the  first  flash  in  Figs.  107  and  108,  it  will  be  seen 
that  the  curves  resemble  closely  corresponding  curves  obtained  with  Sidot 
blende.  The  maximum  intensity  increases  with  length  of  excitation  and 
occurs  later  and  later.  The  areas  included  between  the  curves  and  thecoor- 


300  -*50  600 

Seconds 
Fig.  109. 

Maximum  intensity  of  the  first  flash  plotted  against  length  of  excitation. 
Curve  i,    temp,  of  furnace  339°  C.  (from  Fig.  108). 

2,  277°        (from  Fig.  107). 

3,  "        "       222° 

4,  148° 

dinate  axes  increase  with  the  excitation.  The  effect  of  saturation  is  shown 
both  by  the  change  in  areas  and  by  the  change  in  the  maximum  intensities. 
The  observations  have  been  plotted  in  Fig.  109  so  as  to  show  the  relation 
between  maximum  intensities  and  lengths  of  excitation,  in  which  case  if  one 
considers  the  first  flash  alone  the  curves  are  similar  to  those  already  given 
for  Sidot  blende.  The  effect  of  changing  the  temperature  of  the  furnace 
is  shown  in  Fig.  no. 


Fig.  i 10. 


Maximum  intensity  of  the  first  flash  plotted   against 
temperature  of  the  furnace. 
Curve  A .  exc  ted  600  sec. 
300 
150 
60 
30 


B, 
C. 
D. 
E. 
F. 


«3 


o  100°          aoov         300° 

The  second  flash  does  not  always  follow  the  laws  of  the  first  flash,  as  can 
be  seen  in  Fig.  108.  As  the  length  of  excitation  is  increased,  the  maximum 
intensity  of  the  first  flash  occurs  later  and  later,  while  the  maximum  of  the 
second  flash  occurs  earlier  and  earlier.  This  effect  is  not  well  defined  in 
Fig.  107,  due  perhaps  to  the  fact  that  the  flash  is  of  smaller  intensity  and 
consequently  more  difficult  to  follow  accurately.  The  effect  of  saturation 


DECAY   OF   PHOSPHORESCENCE    PRODUCED   BY    HEATING. 


105 


in  the  case  of  the  second  flash  is  shown  in  Fig.  in.  These  curves  indicate 
that  an  increase  in  the  length  of  excitation  beyond  a  certain  length  does 
not  increase  the  maximum  intensity,  but  decreases  it. 


600 


Fig.  in. 


Maximum  intensity  of  the  second  flash  plotted  against  length  of  excitation. 
Curve  A,  temp,  of  furnace,  339°  C.  (from  Fig.  108). 

B.  "       "  277°        (from  Fig.  107). 

C,  "       "  222° 

The  relation  between  the  maximum  intensity  of  the  second  flash  and 
the  temperature  is  shown  in  Fig.  112.  This  figure  corresponds  closely  to 
Fig.  no,  which  shows  the  corresponding  relation  for  the  first  flash. 

Fig.  113  shows  the  effect  of  delay  in  heating  and  Fig.  114  the  same 


100° 


Fig.  112. 

Maximum  intensity  of  the  second  flash  plotted  against 
temperature  of  the  furnace. 
Curve  B.  excited  300  sec. 

C,  "       150 

D,  "         60 

E.  "         30 

F.  "         15 


300* 


300 


Fig.  113- 

Effect  of  delay  in  heating.  Time  measured  from  end  of  excitation.  Excited  60  seconds  at  room  temperature. 
Temperature  of  furnace,  228°  C.  Curve  A,  decay  at  room  temperature.  The  time  between  the  end 
of  excitation  and  the  beginning  of  heating  is  as  follows:  Curve  B,  2.1  seconds;  C,  25.6;  D,  101.7;  E 
203.2;  F.  401.7;  G,  801.7. 


io6 


STUDIES   IN   LUMINESCENCE. 


curves  plotted  with  time  measured  from  the  beginning  of  heating.  Here 
again,  if  one  considers  the  first  flash  alone,  the  curves  are  similar  to  those 
shown  in  the  case  of  Sidot  blende.  The  longer  the  delay  in  heating,  the 
less  intense  the  flash  and  the  later  the  maximum  intensity  of  each  flash. 
Furthermore,  the  time  of  decay  at  one  temperature  and  constant  length 
of  excitation  is  constant.  It  is  difficult  to  make  any  deduction  from  the 
points  representing  the  second  flash. 

Another  set  of  curves  similar  to  those  shown  in  Fig.  1 13  is  shown  in  Fig. 
115.  These  curves  show  that  the  second  flash  becomes  relatively  larger 
with  respect  to  the  first  as  the  excitation  is  shortened,  and  under  suitable 
conditions,  as  in  curve  C,  may  become  considerably  larger  than  the  first. 


I2Q  /60 

Fig.  114. 


200 


Sec. 


Effect  of  delay  in  heating.     Time  measured  from  the  beginning  of  heating.     .Same  curves  shown  in  Fig.  1 13. 

It  is  difficult  to  say  how  much  difference  exists  between  the  behavior  of 
Sidot  blende  and  Balmain's  paint.  The  decay  curves  at  room  temperature 
are  very  much  alike.  As  the  temperature  is  raised  both  decay  curves 
become  straight  lines,  but  Balmain's  paint  shows  a  transition  through  a 
double  curvature  decay  before  reaching  the  straight  line  decay,  while  Sidot 
blende  does  not  exhibit  this  phenomenon.  Above  the  temperature  at 
which  the  decay  curve  becomes  straight,  Balmain's  paint  shows  a  decay 
approximating  the  decay  at  room  temperature,  while  in  the  case  of  Sidot 
blende  the  decay  is  too  rapid  to  be  followed  with  the  available  apparatus. 
The  decay  of  Sidot  blende  is  so  rapid  that  one  can  not  get  enough  points 


DECAY   OF   PHOSPHORESCENCE   PRODUCED   BY   HEATING. 


107 


on  a  curve  to  say  positively  that  it  does  not  exhibit  double  curvature  at 
high  temperatures.  In  the  cases  where  the  powder  is  heated  after  excita- 
tion, Balmain's  paint  shows  a  double  flash  under  some  conditions,  and  one 
flash  under  other  conditions,  while  Sidot  blende  shows  one  flash  under  all 
conditions,  unless  it  be  admitted  that  there  are  indications  of  a  double  flash 
in  some  of  the  Sidot-blende  curves,  in  which  case  the  twro  substances  show 
substantially  the  same  curves.  Considering  the  first  flash  only,  there  is 


/2 


300 

Fig.  115. 


Sec. 


Effect  of  delay  in  heating.     Time  measured  from  the  end  of  excitation.     Excited 
30  seconds  at  room  temperature.     Temperature  of  furnace,  338°  C. 
Curve  A,  decay  at  room  temp. 

B,  waited  2  sec.  after  excitation  before  heating. 

C,  waited  400  sec.  after  excitation  before  heating. 

perfect  agreement  between  the  curves  of  Sidot  blende  and  Balmain's  paint. 
The  action  of  infra-red  is  more  marked  in  the  case  of  Sidot  blende,  but 
otherwise  there  is  no  marked  difference.  The  fluorescence  spectrum  shows 
two  bands  in  the  case  of  each  substance,  but  there  is  one  point  of  difference, 
in  that  the  band  of  shorter  wave-length  is  more  prominent  in  the  case  of 
Balmain's  paint  and  less  prominent  in  the  case  of  Sidot  blende  than  the 
band  of  longer  wave-length. 


CHAPTER  VII. 

STUDIES  OF  PHOSPHORESCENCE  OF  SHORT  DURATION. 

By  the  methods  described  in  Chapters  IV  and  VI  of  this  memoir  it  is 
difficult  to  make  observations  of  phosphorescence  within  less  than  0.4 
second  of  the  close  of  excitation.  This  suffices  for  the  determination  of 
the  form  of  the  curve  of  decay  writh  the  exception  of  the  region  very  near 
the  origin.  There  are,  however,  many  substances  exhibiting  brilliant  initial 
phosphorescence  where  the  effect  fades  to  unmeasurably  small  intensity 
within  a  few  hundredths  of  a  second. 

It  is  of  considerable  interest  and  importance  not  only  to  determine  the 
curves  of  decay  of  such  substances,  but  also,  since  Lenard1  in  a  recent  paper 
has  questioned  the  linear  relation  between  /"*  and  time  during  the  so-called 
first  process  of  decay,  to  study  the  earliest  portions  of  the  first  process  in 
the  case  of  phosphorescence  of  slow  decay. 


n 


|     1 


B 


M 


0 


1 

•««•• 

C 

V 

,.,I.MI            1 

1     ,U'   or- 
\    \      '    / 

>! 

_J-tl 

Fig.  116. 

Messrs.  Waggoner  and  Zeller  at  our  suggestion  have  investigated  these 
questions  at  some  length,  using  a  new  type  of  phosphoroscope  of  especial 
design  and  working  independently  of  each  other.  The  present  chapter 
contains  a  summary  of  their  researches. 

DR.  WAGGONER'S  STUDIES  IN  PHOSPHORESCENCE  OF  SHORT  DURATION.2 
METHODS  OF  MEASUREMENT. 

The  phosphoroscope  used  in  these  measurements  is  shown  in  Fig.  116. 
It  consists  of  a  disk  D,  fastened  to  a  cylinder  L,  rotating  about  a  horizontal 
axis.  On  the  inside  of  L  is  a  shaft  K  which  rotates  at  the  same  speed  as 
the  cylinder.  On  the  end  of  K  is  a  plane  mirror,  placed  45°  to  the  axis  of 
the  shaft,  and  by  use  of  the  mechanism  at  C  the  position  of  the  mirror, 
relative  to  any  point  on  the  disk  D,  may  be  shifted  while  the  disk  is  rotating. 
The  disk  has  an  opening  at  0,  through  which  the  light  from  the  spark  E 
may  pass  at  each  successive  revolution  and  excite  the  specimen  placed  at  F. 
If  the  mirror  is  in  the  position  shown  it  will  reflect  into  the  slit  of  a  spec- 


'Lenard,  Annalen  der  Physik,  xxi,  p.  641,  1910. 


2C.  W.  Waggoner,  Physical  Review,  xxvn,  p.  209. 

IO9 


110  STUDIES   IN   LUMINESCENCE. 

trophotometer  the  light  which  comes  from  the  phosphorescent  screen  F 
while  it  is  being  excited  by  the  spark.  By  moving  the  rod  R  the  mirror 
is  turned  so  as  to  reflect  the  light  from  the  screen  F  into  the  spectrophoto- 
meter  some  time  after  excitation,  and  in  this  way  the  intensity  of  the  phos- 
phorescence after  successive  excitations  may  be  measured  by  the  spectro- 
photometer  AB.  The  cylinder  L  is  driven  by  a  motor  belted  to  it  over  the 
pulley  P.  S  is  a  worm  driving  a  cog  wheel,  which  serves  as  a  device  for 
recording  the  speed  by  making  an  electric  connection  with  a  chronograph 
once  in  every  hundred  revolutions  of  the  disk.  Current  for  the  motor 
was  taken  from  a  direct-current  source  having  a  special  device  for  main- 
taining constant  voltage,  and  the  speed  of  the  motor  was  found  to  remain 
almost  constant.  The  disk  D  has  a  second  half-disk  fastened  to  the  first 
(not  shown  in  the  figure)  by  which  it  is  possible  to  change  the  size  of  the 
opening  at  O.  The  essential  advantage  of  this  phosphoroscope  over  others 
is  that  the  decay  of  the  phosphorescent  light  may  be  studied  without  chang- 
ing the  time  of  excitation.  Another  feature  of  this  method  is  that  settings 
on  the  spectrophotometer  for  any  curve  may  be  repeated  as  often  as  desired 
and  the  time  taken  for  the  determination  of  each  setting  may  be  as  long 
as  desired.  The  results  shown  in  the  curves  that  follow  were  determined 
from  two  different  settings  of  the  spectrophotometer  for  each  point,  and 
the  plotted  point  represents  the  average. 

The  source  of  excitation  was  the  spark  between  iron  terminals.  To 
produce  this  spark  an  induction  coil  was  connected  to  a  source  of  alternating 
current  of  60  cycles  frequency,  a  small  condenser  being  connected  in  mul- 
tiple with  the  spark  gap.  In  order  to  study  the  effect  of  decreased  time  of 
excitation,  it  was  found  necessary  to  increase  the  frequency  of  the  spark, 
this  being  accomplished  by  connecting  it  to  a  source  of  alternating  current 
giving  140  cycles.  With  this  higher  frequency  the  chances  of  the  spark 
exciting  the  screen  at  each  turn  of  the  disk  is  greater ;  the  higher  frequency 
spark  was  also  used  as  a  check  on  the  curves  taken  at  the  lower  frequency. 

The  light  from  an  incandescent  lamp  connected  to  a  constant  potential 
source  was  used  for  comparison  in  the  spectrophotometric  measurements. 

METHODS  OF  PREPARING  THE   PHOSPHORESCENT   COMPOUNDS. 

The  phosphoroscope  just  described,  although  it  has  a  considerable  range 
as  to  speed,  is  especially  adapted  to  compounds  whose  phosphorescence 
decays  rapidly.  A  number  of  such  compounds  were  prepared  and  were 
studied  along  with  several  specimens  of  willemite  whose  decay  was  of  suit- 
able rapidity. 

In  preparing  the  compounds  the  methods  given  by  Wiedemann  and 
Schmidt1  and  Andrews2  were  followed.  The  ease  with  which  these  com- 
pounds may  be  prepared  and  the  intensity  of  the  phosphorescent  light 
given  off  when  they  are  excited  by  the  spark  seem  to  warrant  a  rather 
detailed  account  of  their  preparation. 

ZnClz-\-MnS04.3 — Some  zinc  chloride  prepared  from  metallic  zinc  was 
dissolved  in  a  small  quantity  of  distilled  water.  A  small  trace  (usually  less 

»Wied.  Ann.,  54,  p.  604,  1895.  'Science,  xix,  March  1904. 

*MnSO«  was  selected  as  an  active  salt  in  a  number  of  cases,  since  it  has  been  found  by  Lenard  and  Klatt 
(Annalen  der  Physik,  15,  p.  243,  1904)  that  with  this  salt  the  intensity  of  phosphorescence  is  less  affected 
by  varying  the  concentration  than  with  any  other  active  agent. 


STUDIES   OF    PHOSPHORESCENCE   OF   SHORT   DURATION.  Ill 


than  i  per  cent)  of  MnSO4,  dissolved  in  water,  was  added  to  the 
solution  and  the  whole  brought  to  the  boiling  point.  An  equal  volume  of 
sodium  silicate,  as  the  flux,  was  then  added  and  the  whole  evaporated  to 
dry  ness.  On  being  exposed  to  the  spark,  faint  green  phosphorescence 
could  be  seen.  It  was  then  placed  in  a  porcelain  crucible  and  heated  to  a 
bright  red  for  2  hours.  When  cool  it  showed  pale  green  fluorescence  when 
exposed  to  the  spark,  and  when  the  exciting  source  was  cut  off  it  was  found 
to  have  considerable  bright  green  phosphorescence.  A  sample  of  the 
powder  at  this  point  was  kept  and  marked  ZnCl2  No.  i.  The  remainder 
of  the  powder  was  heated  to  a  bright  red  for  3  hours  and  when  cool  showed 
both  fluorescence  and  phosphorescence  of  greater  intensity  than  did  ZnCl2 
No.  i.  A  sample  of  this  was  saved  and  marked  No.  2.  The  remaining 
powder  was  heated  2  hours  at  a  bright  red;  on  examination  when  cool  it 
was  found  to  have  lost  some  of  its  original  phosphorescent  intensity.  This 
was  marked  ZnCl2  No.  3. 

CdCli+xMnSOi—  Substituting  CdCl2  for  ZnCl2,  but  giving  it  exactly 
the  same  treatment,  it  was  found  that  it  was  necessary  to  heat  the  compound 
at  a  bright  red  for  3  hours  before  the  phosphorescence  was  very  intense,  and 
that  subsequent  heating  had  no  marked  effect  on  its  phosphorescent  in- 
tensity when  exposed  to  the  spark.  The  fluorescence  was  pink  and  the 
phosphorescence  dark  red. 

CdSOi+xMnSOi.  —  Some  CdSO4  was  dissolved  in  water  with  a  small 
trace  of  MnSO4  and  the  whole  heated  to  dryness  without  adding  the  flux. 
The  resulting  white  powder  showed  pale  yellow  fluorescence,  and  an  orange 
yellow  phosphorescence  of  much  longer  duration  than  the  zinc  compounds. 

ZnSOt+xMnSOt—  ZnSO4  substituted  for  CdSO4  and  treated  in  the 
same  way  resulted  in  a  white  powder  which  showed  pink  fluorescence  and 
bright  red  phosphorescence,  but  the  intensity  was  too  small  to  be  measured 
by  the  method  used. 

Some  "chemically  pure"  calcium  sulphide  was  purchased  with  the  view 
of  trying  to  prepare  compounds  of  CaS  and  MnSO4,  but  it  was  found  to  be 
already  an  active  phosphorescent  compound  having  a  brownish-red  color. 

Two  large  pieces  of  willemite,  showing  brilliant  green  phosphorescence 
with  the  iron  spark,  were  studied.  It  was  found  that  their  rates  of  decay 
were  not  the  same.  The  sample  having  the  slowest  decay  was  marked 
willemite  No.  5.  The  sample  having  the  very  rapid  decay  was  crushed 
into  a  fine  powder,  a  sample  made  into  a  screen  and  marked  willemite  No.  i  . 
Part  of  the  powder  was  heated  to  a  bright  red  heat  in  a  porcelain  crucible  for 
45  minutes.  This  was  marked  willemite  No.  2.  The  remainder  of  the 
powder  was  heated  for  2  hours  at  a  bright  red,  and  marked  willemite  No.  3. 

One  ZnCl2+o;MnSO4  sample  made  by  W.  S.  Andrews,  Schenectady, 
N.  Y.,  was  also  studied. 

For  purposes  of  study,  screens  were  made  by  placing  the  phosphorescent 
powder  on  small  squares  of  heavy  dark  brown  cardboard,  the  cards  having 
been  covered  first  with  white  "zapon"  varnish. 

Among  the  preparations  just  described  was  one  of  cadmium  sulphate 
and  manganese  sulphate  made  by  simply  evaporating  to  dryness  a  water 
solution  of  the  two  salts.  After  studying  this  compound  it  was  put  aside 
for  several  months  and  when  taken  up  later  for  further  work  it  had  lost 


112  STUDIES   IN    LUMINESCENCE. 

its  power  of  phosphorescence  under  excitation  by  the  iron  spark,  but  on 
being  heated  it  regained  its  former  brilliancy.  This  led  to  the  suspicion  that 
these  simple  salts  are  photo-luminescent  only  when  in  the  anhydrous  condi- 
tion, and  subsequent  tests  showed  this  to  be  true  of  all  the  compounds  studied. 

The  fact  that  moisture  affects  the  photo-luminescence  of  cadmium  salts 
may  be  shown  by  exciting  the  anhydrous  phosphorescent  compound  by  an 
ultra-violet  source  and  allowing  the  light  to  decay  for  several  seconds. 
Now  if  a  drop  of  water  be  placed  on  the  decaying  compound  the  residual 
light  will  flash  out  brilliantly  for  an  instant  and  then  disappear  very  rapidly. 
It  was  first  thought  that  this  phenomenon  was  due  to  the  changes  which 
take  place  within  the  salt  as  it  takes  up  water  of  crystallization.  The 
thermo-phosphorescence  of  the  cadmium  compounds,  however,  is  quite 
marked,  and  Dr.  Kortright,  of  the  Department  of  Chemistry,  West  Vir- 
ginia University,  suggested  that  the  effect  might  be  partly  due  to  thermo- 
phosphorescence  brought  about  by  the  heating  which  accompanies  the 
process  of  hydration.  To  obtain  some  idea  of  the  rise  in  temperature, 
several  grams  of  anhydrous  salt  were  placed  in  a  test-tube  with  a  ther- 
mometer, and  water  was  added.  The  rise  of  temperature  was  found  to  be  as 
much  as  5°  C.,  which  would  account  in  a  large  measure  for  the  phenomenon 
purely  on  the  basis  of  thermo-phosphorescence.  Alcohol,  which  is  not  a 
solvent  for  cadmium  sulphate,  was  tried,  but  gave  no  effect. 

The  fact  that  these  cadmium  compounds  do  not  show  photo-luminescence 
when  in  the  crystalline  state  led  the  writer  to  try  some  of  the  original  cad- 
mium sulphate  from  which  the  compounds  with  manganese  were  made ;  and 
when  the  water  of  crystallization  was  driven  off,  the  salt  was  found  to  be 
highly  phosphorescent.  The  color  of  the  phosphorescence  was  similar  to 
that  when  a  manganese  salt  had  been  added,  but  was  less  intense.  When 
subjected  to  kathode  rays  the  sulphate  showed  more  brilliant  fluorescence 
but  less  phosphorescence  than  when  excited  by  the  spark. 

An  attempt  was  first  made  to  purify  the  original  cadmium  sulphate  by 
forcing  it  out  of  water  solution  with  sulphuric  acid,  but  this  treatment 
increased  rather  than  decreased  the  intensity  of  phosphorescence. 

Another  method  was  to  precipitate  the  cadmium  with  hydrogen  sulphide 
as  cadmium  sulphide,  wash  it  with  water  till  free  from  sulphates,  then  re- 
dissolve  the  sulphide  in  sulphuric  acid,  crystallizing  the  salt  formed.  This 
sulphate  still  gave  a  brilliant  yellow  phosphorescence  under  both  the  iron 
spark  and  the  kathode  rays. 

The  next  method  was  to  crystallize  the  salts  in  fractions  from  water 
which  was  slightly  acidified  with  sulphuric  acid  to  prevent  hydrolysis. 
The  mother  liquor  was  saved  in  this  case  and  the  crystals  redissolved  and 
crystallized  out.  It  was  found  that  the  phosphorescence  of  the  successive 
crystals,  as  they  separated  out  from  the  retained  mother  liquor,  decreased  rap- 
idly in  intensity  and  at  the  end  of  the  seventh  fractionalization  was  almost 
nil,  thus  indicating  the  presence  of  an  impurity  less  soluble  than  the  cadmium 
sulphate.  This  view  was  supported  by  a  micro-chemical  test  made  by  Dr. 
Wilkinson1  showing  the  presence  of  sodium  and  a  slight  trace  of  zinc. 

In  casting  about  for  a  cadmium  sulphate  free  from  phosphorescence 
numerous  samples  were  tried  and  it  was  found  that  the  "tested  purity" 
cadmium  sulphate  made  by  Eimer  and  Amend  was  free  to  a  remarkable 

'Journal  Physical  Chem.,  xm,  p.  719,  1909. 


STUDIES   OF   PHOSPHORESCENCE   OF   SHORT   DURATION.  113 

degree.  The  phosphorescence,  under  the  iron  spark,  of  the  anhydrous 
powder  was  so  small  that  it  was  necessary  to  stay  in  the  dark  room  a  long 
time  before  the  eye  became  sensitive  enough  to  detect  the  phosphorescence 
at  all ;  and  even  then  one  could  not  be  sure  of  the  color.  The  analysis  of  this 
material  given  on  the  bottle  was  as  follows:  "Cd,  52.84  per  cent;  SOs,  47.15 
per  cent;  As,  none;  Zn,  none."  This  material  was  tested  by  the  kathode 
rays  and  it  was  found  to  show  neither  fluorescence  nor  phosphorescence. 

That  a  zinc  salt,  when  added  to  a  cadmium  salt,  or  vice  versa,  would 
produce  a  phosphorescent  compound  has  been  pointed  out  by  Wiedemann 
and  Schmidt.1  This  fact  led  to  the  suggestion  that  the  presence  of  small 
traces  of  zinc  in  cadmium  salt,  or  vice  versa,  might  be  detected  by  the 
photo-phosphorescence  which  these  compounds  show,  and  some  rough 
tests  were  made  showing  photo-phosphorescence  with  less  than  one  part 
of  zinc  sulphate  in  10,000  parts  of  the  phosphorescent-free  cadmium  sul- 
phate. The  phosphorescent  color  was  green,  but  the  intensity  was  not 
so  great  as  when  a  sodium  salt  is  added  to  a  cadmium  salt. 

The  pure  cadmium  sulphate  is  very  susceptible  to  a  small  impurity; 
how  susceptible  will  appear  from  the  following  experiment:  Some  water, 
which  was  twice  distilled  in  an  all-glass  still,  was  placed  in  a  bottle  and 
allowed  to  stand  over  night.  The  following  day  this  water  was  used  to 
dissolve  some  of  the  pure  cadmium  sulphate  and  was  then  driven  off. 
When  the  cadmium  sulphate  was  tried  under  the  ultra-violet  source  it  was 
found  to  be  highly  phosphorescent.  200  c.c.  of  this  same  water  when 
evaporated  down  in  a  platinum  dish  left  a  residue  so  small  that  it  could 
not  be  seen  except  when  the  platinum  dish  wras  at  red  heat. 

Cadmium  Sulphate-Sodium  Compounds. — Starting  with  the  phosphor- 
escence-free cadmium  sulphate,  a  series  of  preparations  was  made  by  the 
addition  of  various  salts  of  sodium.  The  method  used  was  that  already 
described.  It  was  to  dissolve  the  crystals  of  sodium  salt  and  cadmium 
sulphate  in  water;  slowly  evaporate  the  solution  to  dryness,  and  test  the 
compound  for  phosphorescence,  when  it  reached  the  temperature  of  the 
room,  by  exciting  by  the  iron  spark.  The  compound,  from  the  original 
crystals  to  the  final  test,  was  kept  in  a  large  platinum  dish  and  every  pre- 
caution was  taken  to  prevent  impurities  from  entering  the  solution. 

The  water  was  twice  distilled  in  an  all-glass  still  and  used  immediately 
in  order  to  prevent  it  from  taking  up  sodium  from  the  glass  container. 
To  produce  an  even  and  easily  regulated  temperature  a  small  electric  furnace 
was  made  by  wrapping  "nichrome"  ribbon  around  a  porous  battery  cup; 
this  proved  to  be  a  very  satisfactory  arrangement,  for  the  platinum  vessel 
could  be  placed  inside  of  this  furnace  and  covered  over  lightly  to  prevent 
impurities  entering  from  the  air  while  evaporation  was  taking  place. 

Only  readily  soluble  salts  of  sodium  were  used ;  and  each  one  was  tested 
in  the  anhydrous  condition  for  the  presence  of  phosphorescence.  In  one 
or  two  cases  it  w7as  necessary  to  recrystallize  the  salt  to  free  it  from  the 
impurity  causing  the  phosphorescence.  Several  different  percentages  of 
cadmium  and  sodium  wrere  tried  and  it  was  found  that  the  intensity  of  phos- 
phorescence depended  somewhat  upon  the  percentage  of  sodium  present; 
but  so  far  as  the  tests  were  carried  no  change  was  produced  in  the  color 


'Wied.  Ann.,  vol.  56,  1895. 


114  STUDIES   IN   LUMINESCENCE. 

of  phosphorescence  by  rather  wide  variations  in  the  percentages  of  the  added 
sodium.  In  most  cases  of  the  compounds  recorded  in  Table  17  the  amount 
of  sodium  salt  present  was  not  far  from  i  per  cent. 

An  inspection  of  Table  17  shows  that  the  color  of  the  phosphorescence 
of  cadmium  salts  with  sodium  ranges  from  blue  to  yellow.  In  all  but  two 
cases  the  intensity  was  very  small — so  small  as  to  make  it  impossible  to 
obtain  from  these  compounds  reliable  curves  of  decay  with  a  spectro- 
photometer.  Doubtless  all  of  these  could  be  increased  in  intensity  by  find- 
ing the  suitable  proportion  of  sodium. 

TABLE  17. 


Sodium  salt  added. 

Phosphorescent  color. 

Spectrum. 

Na2SO4 
Na^O, 
Na2SiO3 
NaHPO4 
NaNO, 
Na2MnO4 
NaFl 
Na,CrO4 
NaOH 

Yellow. 
No  phosphorescence. 
Blue. 
Faint  green. 
Faint  yellow. 
Greenish  yellow. 
Greenish  yellow. 
No  phosphorescence. 
No  phosphorescence. 

o  .  486/1,  max  ...510/1,  o  .  604/1 

.  522/1,  max.  ..  566/1,     .616/1 
.474/1,  max.  ..  574/1,      .612/1 

IsiaCl 
NaClO3 
Na.CO\ 
NaBr 
Na2B4O7 
Na2CrO7 
Na2SO4Al2(SO4)3 

Faint  yellow. 
No  phosphorescence. 
Blue. 
Pale  green. 
No  phosphorescence. 
Faint  green. 

.414/1,  max.  .  .480/1,     .600/1 

DISCUSSION   OF  RESULTS. 

The  results  of  the  study  of  the  substances  just  described  are  indicated 
by  means  of  decay  curves  plotted,  like  many  of  those  which  appear  in 
Chapters  IV  and  VI,  with  I~-  as  ordinates,  and  times,  counted  from  the 
close  of  excitation,  as  abscissas.  It  will  be  seen  that  the  curves  have  the 
same  general  characteristics  as  those  obtained  by  the  measurement  of 
long-time  phosphorescence.  They  may  be  regarded  as  consisting  of  two 
straight  lines  merging  into  one  another. 

The  curves  in  Fig.  117  are  decay  curves  typical  of  three  of  the  substances 
studied.  In  the  case  of  curve  A  (willemite  No.  5)  the  decay  was  so  rapid 
that  it  was  difficult  on  account  of  the  small  intensity  of  the  light  to  follow 
the  "second  process"  very  far. 

In  each  curve  one  point  has  been  plotted  to  the  left  of  the  zero  on  the 
ordinates.  This  observation  is  made  on  the  fluorescent  light,  i.  e.,  it  is  the 
intensity  of  the  light  coming  from  the  screen  when  the  mirror  is  in  position 
to  reflect  the  light  which  comes  from  the  screen  during  excitation.  The 
zero  point,  or  the  point  where  the  exciting  light  fails  to  be  reflected  into 
the  spectrophotometer,  was  determined  by  placing  a  piece  of  white  paper 
in  place  of  the  screen. 

Since  the  induction  coil  furnished  at  most  120  discharges  per  second  it 
will  be  seen  that,  for  the  time  of  excitation  employed  in  the  measurements 
illustrated  inFig.  1 17,  three  sparks  per  revolution  of  thedisk  is  themaximum 


STUDIES   OF   PHOSPHORESCENCE   OF   SHORT  DURATION. 


number  that  could  pass  while  the  screen  is  exposed.  It  would  seem  then  that 
the  excitation  might  be  uncertain  and  irregular.  These  excitations,  how- 
ever, follow  each  other  very  rapidly;  for  example,  in  curve  B,  Fig.  117,  the 
rotating  disk  makes  a  complete  revolution  in  0.108  second,  and  since  10  to 
20  seconds  were  required  to  make  a  setting  of  the  spectrophotometer,  what- 
ever changes  take  place  in  the  spark  during  successive  excitation,  the  reading 
is  an  average  value  of  the  intensity  at  that  point.  In  order  to  make  sure 
of  this,  readings  were  taken  starting  with  the  lowest  intensity  and  ending 
with  the  highest,  then  starting  with  the  highest  and  following  the  decay  to 
the  lowest  value  of  intensity,  the  points  plotted  being  an  average  of  the  two 
settings  for  each  point  on  the  curve. 

If  the  substances  here  studied  exhibit  the  same  hysteresis  phenomena 
that  have  been  found  in  Sidot  blende  and  several  other  substances,1  the 
decay  curves  obtained  by  this 
method  are  probably  not  the  same 
as  would  be  observed  if  it  were 
possible  to  restore  the  substance 
to  a  standard  condition  between 
successive  periods  of  excitation, 
for  example  by  the  action  of  infra- 
red rays.  In  using  these  curves 
to  test  any  theory  of  the  decay  of 
phosphorescence,  this  fact  must 
of  course  be  taken  into  considera- 
tion. 

The  curves  in  Fig.  1 1 8  indicate 
a  much  slower  decay  than  those 
shown  in  Fig.  117.  The  initial 
decay  or  first  process,  however, 
is  quite  rapid.  These  curves  were 
more  difficult  to  obtain,  since  the 
maximum  of  intensity  lay  in  the 
yellow-red,  while  the  substances 
whose  curves  are  given  in  Fig. 
1 1 7  have  a  green  phosphorescence. 
On  this  account  the  points  are  less 


40 


7 


? 


7 


04 

Seconds 


.06 


Fig.  1 1 7 — Decay  curves. 


Curve  A,  Willemite  No.    5,  time   of  excitation  0.026 

second. 
Curve  B,  Andrews  ZnCh,  time  of  excitation  0.027 

second. 
Curve  C,  ZnCh  No.  3,  time  of  excitation  0.26  second. 

accurately    determined    than    in 

Fig.   117.     It  is  evident,   nevertheless,  that  the  curves  have  the  same 

general  character  and  consist  of  two  straight  lines  merging  into  each  other. 

All  the  substances  mentioned  in  this  chapter  are  excited  to  fluorescence 
by  kathode  rays.  The  mixture  containing  CdSO4  was  especially  brilliant 
and  exhibited  kathodo-luminescence  of  an  intense  yellow  color.  They 
were  also  excited  to  some  extent  by  X-rays,  the  CdSO4  again  showing  the 
greatest  intensity.  In  all  cases  the  phosphorescence  excited  by  X-rays 
was  too  small  to  be  measured  with  the  present  apparatus. 

The  curves  given  in  Fig.  119  show  the  change  in  the  decay  of  willemite 
after  heating.  Curve  L  shows  the  decay  of  the  untreated  willemite. 
Curve  M  shows  the  decay  after  heating  the  willemite  for  45  minutes  at  a 


'See  Chapters  IV  and  V. 


n6 


STUDIES   IN   LUMINESCENCE. 


bright  red  heat.  It  is  clear  from  the  curves  that  the  heat  treatment  has 
decreased  the  initial  intensity  and  made  the  decay  less  rapid;  both  these 
changes  may  be  observed  by  the  eye  alone  if  the  two  screens  are  excited 
in  a  dark  room.  Willemite  No.  3  which  had  been  heated  2  hours  was  found 
to  give  a  curve  whose  points  fell  so  nearly  on  curve  M  that  it  was  not  plotted. 
The  effect  of  heat  upon  the  decay  in  ZnCl2+^MnSO4  is  similarly 
shown  in  Fig.  120.  Curve  H  represents  the  decay  in  a  sample  of  the  sub- 
stance taken  as  a  dry  powder,  which  was  prepared  by  heating  the  mixture 
only  2  hours  at  a  bright  red.  Curve  K  represents  the  decay  of  a  sample 
heated  5  hours,  and  curve  G  after  7  hours  heating.  The  curves  show  that 
the  first  heating  was  not  sufficient  to  bring  out  the  initial  brightness  and 
longer  decay  of  the  substance,  and  that  there  is  a  time  limit  to  the  heating 


Fig.  1 1 8. 

Showing  the  decay  in  different  substances. 
Curve  D,  CaS,       time  of  excitation  o .  033  sec. 

E.  CdSOi.      "      "  0.032 

F,  CdCb,       "      "          "          0.031 


0        .01       .OZ       .03       .04      .05 

Seconds 
Fig.  119- 

Showing  effect  of  the  heat  treatment. 

Curve  L,  willemite  No.  i,  time  of  excita- 
tion 0.022  sec. 

Curve  M,  willemite  No.  2,  time  of  excita- 
tion 0.022  sec. 


necessary  to  produce  maximum  intensity;  for  on  heating  beyond  this  the 
initial  intensity  becomes  less.  From  the  behavior  of  this  artificial  com- 
pound it  would  seem  that  the  natural  willemite  shown  in  Fig.  119  had 
reached  a  maximum  heat  treatment  already,  for  further  heating  decreased 
its  initial  intensity.  The  slope  of  the  curves  in  Fig.  1 20  indicate  little  change 
in  the  rate  of  decay  after  the  initial  drop ;  unfortunately  the  intensity  be- 
comes so  small  that  it  was  found  impossible  to  carry  the  curves  farther. 
The  readings  in  this  region  represent  the  average  of  a  number  of  settings. 

The  curves  given  in  Fig.  121  show  the  effect  of  changing  the  time  of 
excitation  in  ZnCl2  No.  3.  This  was  accomplished  by  making  the  opening 
of  the  disk  in  Fig.  1 1 6  smaller.  It  was  found  that  when  the  6o-cycle  current 
was  used  for  the  spark  the  curves  taken  with  decreased  time  of  excitation 


STUDIES   OF   PHOSPHORESCENCE    OF   SHORT   DURATION. 


117 


were  more  or  less  irregular.  This  was  no  doubt  due  to  the  fact  that  with 
the  small  opening  of  the  disk  the  irregularities  of  the  spark  were  more  pro- 
nounced. However,  on  the  i4O-cycle  current  the  curves  were  quite  regular 
and  could  be  duplicated  very  closely.  The  curves  given  in  Fig.  121  were 
taken  with  the  spark  operated  from  the  i4o-cycle  current.  It  will  be  noted 
that  a  decrease  in  the  time  of  excitation  brings  about  a  more  rapid  decay. 

The  curves  in  Fig.  121  also  serve  to  confirm  the  results  obtained  with  the 
6o-cycle  current ;  for  it  will  be  seen  that  they  have  the  same  general  shape 
as  those  given  for  ZnCl2  No.  3  in  Fig.  117. 


45 
43 
4-1 

^ 
37 

35 
33 
31 
29 


0   .01   .02   .03   .04   .05   .06   .07 

Seconds 
Fig.  120. 


.01   .02   .03   .04   .05 

Seconds 
Fig.  121. 


Curves  showing  effect  of  heat  treatment.  Showing  the  effect  of  changing  the  time  of  excitation. 

Curve  H,  ZnCh  No.  i.  time  of  excitation  0.031  sec.    Curve  A,  ZnCh  No.  3.  time  of  excitation  0.013  sec- 
K>  "    2,    "  0.031  sec.  B,        "          "         "     "  "         0.024  =>ec- 


G, 


C, 


0.058  sec. 


EFFECT  OF   INFRA-RED  ON  THE   INITIAL,  DECAY  OF  SIDOT  BLENDE. 

The  effect  of  infra-red  on  long-time  phosphorescence  is  well  known,1  and 
it  was  natural  to  expect  that  short-time  phosphorescence  would  be  affected 
in  somewhat  the  same  way.  The  method  of  experimentation  followed  was 
to  allow  the  light  from  the  arc  to  fall  on  the  phosphorescent  screen  through 
a  piece  of  very  dense  ruby  glass,  and  to  compare  the  shape  of  the  decay  curves 
taken  with  and  without  the  infra-red.  It  was  found,  however,  that  none 
of  the  short-time  substances  were  affected  in  measurable  amount.  There 
was  a  slight  indication  of  some  change  in  the  shape  of  the  later  portion  of 
the  decay  curve,  but  the  error  in  setting  the  spectrophotometer  in  this 
region,  where  the  light  is  so  faint,  would  account  for  the  observed  change. 

While  the  effect  of  infra-red  on  the  decay  of  short-time  phosphorescent 
compounds  is  so  small  that  it  can  not  be  readily  measured,  the  effect  of 
infra-red  on  Sidot  blende  is  well  marked. 


'See  Chapter  V  of  this  treatise. 


n8 


STUDIES   IN   LUMINESCENCE. 


In  Fig.  122,  curve  B,  showing  the  decay  under  the  action  of  the  infra-red, 
is  the  average  of  a  number  of  determinations  and  the  shape  of  the  curve  is 
fairly  definite.  The  data  for  curve  .4  are,  however,  less  reliable.  There  is 
some  doubt  also  as  to  the  shape  of  the  curve  during  the  first  hundredth  of 

a  second.  It  was  neces- 
sary to  change  the  posi- 
tion of  the  disk  on  the 
shaft  of  the  phosphoro- 
scope  in  studying  the 
effect  of  infra-red  and  it 
is  possible  that  the  zero 
given  may  be  incorrect, 
i.  e.,  the  points  on  the 
zero  ordinate  and  per- 
haps the  one  following 
may  be  in  the  fluorescent 
light,  and  therefore  the 
real  phosphorescent  de- 
cay may  have  started  at 


.01       .02       .03 


.04       .05      .06 

Seconds 


.07      .08     .09 


Fig.  122. 


Curves  showing  effect  of  infra-red  on  initial  decay  of  Sidot  blende. 
Curve  .4,  without  infra-red,  time  of  excitation  0.04  second. 
Curve  B,  with  infra-red,  time  of  excitation  0.04  second. 


some  later  time  than 
the  zero  given.  This 
possible  error  in  the  zero 

only  applies  to  the  curves  taken  in  studying  the  infra-red  and  in  no  way 

affects  the  other  curves. 

There  is  no  question  as  to  the  effect  of  infra-red  on  the  initial  decay  of 

Sidot  blende,  but  owing  to  the  very  slow  decay  the  shape  of  the  curves 

may  be  more  or  less  in  error. 


DECAY   CURVES   FOR   DIFFERENT   WAVE-LENGTHS. 

In  this  determination  the  curves  of  decay  were  taken  for  different  wave- 
lengths of  the  phosphorescence  spectnim  of  several  substances.  So  far 
as  could  be  seen  with  the  spectroscope  all  these  spectra,  at  room  temperature, 
consisted  of  a  single  band,  the  location  and  extent  of  which  in  each  case  is 
indicated  in  Table  18. 


CaS 
CdCl2 
CdSO4 
ZnCl2  No.  i 
Andrews'  ZnCl2 
Willemite  No.  i 


TABLE  18. 


0.53    to  0.64  /z  w  thmax.  at  O-575/x 


•  545 

•  5H 
•49 
•49 
.46 


•65  M 
.623/1 

•  585M 

•  545M 

•  56  M 


•  59  M 

•  565M 

•  52  IJL 

•  52  M 

•  52  M 


To  determine  the  rate  of  decay  of  different  parts  of  the  band  for  any  sub- 
stance, two  methods  were  used.  One  was  to  take  a  number  of  decay  curves 
at  different  wave-lengths  of  the  band,  and  to  plot  a  curve  showing  the  inten- 
sity, at  a  fixed  time  after  excitation,  for  the  different  wave-lengths.  In  the 
second  method  the  mirror  of  the  phosphoroscope  was  fixed  so  as  to  reflect  the 
light  from  the  screen  a  certain  time  after  excitation;  then,  by  shifting  the 
telescope  of  the  spectrophotometer,  the  intensity  of  the  phosphorescence  was 


STUDIES   OF   PHOSPHORESCENCE   OF   6HORT   DURATION. 


119 


determined  for  the  different  wave-lengths.  The  phosphoroscope  was  then 
adjusted  to  give  the  decay  at  a  later  period,  and  the  intensity  for  different 
wave-lengths  was  measured  in  the  same  way.  Both  of  these  methods  gave 
the  same  results,  i.e.,  that  all  portions  of  the  band  seemed  to  decay  at  the 
same  rate.  The  curves  shown  in  Fig.  123  are  typical  for  all  the  substances 
studied.  In  none  of  the  substances  was  there  an  indication  of  a  shift  of  the 
maximum  of  the  curve  as  the  decay  went  on.  If  any  change  occurs  during 
the  decay  it  is  too  small  to  be  detected  by  the  method  used. 

SPECTROPHOTOMETRIC   STUDY  OF  THE   CADMIUM-SODIUM   COMPOUNDS. 

The  time  of  decay  of  these  substances  is  so  much  longer  than  that  of 
the  others  considered  in  this  chapter  that  the  phosphoroscope  already 
described  was  not  well  suited  to  their  study.  A  new  instrument  was 
therefore  devised  for  the  purpose.  This  machine  is  a  modification  of  the 
phosphoroscope  described  by  Kester,'but  is  so  constructed  that  it  is  possible 

to  determine  curves  of  decay         

without  changing  the  time 
of  excitation. 

Fig.  124  shows  the  top 
view  of  the  apparatus  for 
making  the  decay  curves. 
W  is  an  iron  pulley  45  cm. 
in  diameter  mounted  on  a 
vertical  shaft.  The  mass  of 
the  pulley  being  consider- 
able, it  acted  as  a  balance 
wheel  on  the  driving  motor 
to  keep  the  speed  constant. 
The  speed  was  automati- 
cally recorded  on  a  chrono- 
graph. The  compound  to 
be  studied  was  sifted  lightly 


.58A 


Fig.  123. 

Curve   showing  the  phosphorescence  spectrum  of  ZnCh  No.  i , 
at  different  times  after  the  exciting  light  was  removed. 

over  a  strip  of  paper  covered 

with  "zapon"  varnish;  the  strip  of  paper  was  then  placed  on  the  rim  of 
the  pulley.  The  spark  gap  S  was  mounted  in  such  a  way  that  it  could 
be  moved  to  different  points  on  the  periphery  of  the  wheel  without  in  the 
least  disturbing  the  speed  of  the  wheel  or  the  continuity  of  the  spark.  This 
made  it  possible  to  maintain  a  constant  excitation  and  still  obtain  the  inten- 
sity of  the  phosphorescence  at  various  times  after  excitation.  The  phos- 
phorescent light  to  be  studied  enters  the  Lummer-Brodhun  spectrophoto- 
meter  at  C2.  The  comparison  source  is  shown  at  C\,  and  the  telescope  at  T. 

Fig.  125  shows  the  phosphorescence  spectrum  of  a  cadmium-sodium- 
manganate  preparation  taken  with  the  apparatus  just  described  and  with 
an  incandescent  lamp  as  the  comparison  source. 

Fig.  126  shows  the  phosphorescence  spectrum  of  a  cadmium-manganese- 
chloride  compound.  The  distribution  of  intensities  in  these  two  cases  is 
very  similar  and  the  spectra  correspond  closely  with  that  of  cadmium- 


'Kester:  Physical  Review,  XXH,  p.  280. 


120 


STUDIES   IN    LUMINESCENCE. 


manganese-sulphate  preparation  previously  observed,  which  showed  a 
band  extending  from  0.514  to  0.623  n,  with  a  maximum  at  0.565 /x.  It 
would  seem  from  these  results  that  the  manganese  salts  produce,  when 
added  to  a  cadmium  salt,  a  phosphorescent  compound  whose  spectrum 
shows  the  same  maximum,  and  is  independent  of  the  salt. 


W 


Fig.  124. 
Showing  top  view  of  phosphoroscope. 


f 


Wa 


50  54-  58  62 

Fig.  126. 

Showing  the  phosphorescence  spectrum  of 
a  CdSOi-MnCh  compound,  taken  1.44 
seconds  after  excitation. 


Of  the  sodium  salts  this  is  not  true,  for  the  colors  are  widely  different. 
An  attempt  was  made  to  measure  the  spectrum  of  all  the  sodium  compounds, 
but  in  most  cases  the  intensity  was  far  too  small  to  measure. 


4.7.8 


Z 


53  54.  56  58  60  62 

Fig.  125. 

Showing  the  phosphorescence  spectrum  of  a  Cd-Na2- 
MnOi  compound,  taken  0.374  second  after  excitation. 


55 

* 

45 
35 

25 

0 

o  

1  ' 

,-< 

— 

I 

S^ 

/ 

I 

1 

1                Z              3              4 

Seconds 

Fig.  127. 

Showing  curve  of  decay  of  a  CdSO4-NaBr 
compound. 


Decay  curves  were  taken  from  all  the  compounds  whose  intensity  was 
sufficient  to  measure.  All  these  curves  show  the  same  characteristics  when 
plotted  with  the  reciprocal  of  the  square  root  of  the  intensity  as  ordinates 


STUDIES   OF   PHOSPHORESCENCE    OF    SHORT   DURATION.  121 

and  time  in  seconds  as  abscissas.  Fig.  127,  which  gives  the  decay  curve 
for  cadmium  sulphate  with  sodium  bromide,  and  exhibits  the  two  straight 
lines  merging  into  each  other,  is  a  typical  example. 

SUMMARY. 

The  most  important  points  brought  out  by  the  experiments  here  de- 
scribed may  be  briefly  stated  as  follows : 

1.  The  decay  curve  when  plotted  with  the  values  of  I~*  as  ordinates 
and  corresponding  values  of  t  as  abscissas  consists  of  two  straight  lines 
gradually  merging  into  each  other.     In  this  respect  the  short-time  and 
long-time  phosphorescent  compounds  seem  to  be  similar. 

2.  The  transition  from  fluorescence  to  phosphorescence  is  gradual,  i.  e., 
the  curve  shows  no  sign  of  discontinuity. 

3.  The  shape  of  the  decay  curve  and  the  intensity  depend  upon  the  time 
of  excitation. 

4.  The  effect  of  heat  treatment  is  such  as  to  change  both  the  intensity 
and  the  rate  of  decay  of  phosphorescence. 

5.  The  effect  of  infra-red  on  short- time  phosphorescence,  if  it  exists  at 
all,  is  very  slight;  but  its  effect  on  the  initial  decay  of  Sidot  blende  is  quite 
marked. 

6.  The  experiments  indicate  that  at  ordinary  temperatures  all  portions 
of  the  phosphorescence  band  decay  at  the  same  rate. 

THE  EXPERIMENTS  OF  MR.  CARL  ZELLER.1 

The  experiments  described  below  deal  with  the  phosphorescence  of  three 
groups  of  compounds,  namely,  (i)  the  aniline  dyes  in  their  solid  form;  (2)  a 
group  of  manganese  compounds  of  known  percentage  concentration  and  a 
group  of  cadmium  compounds,  both  prepared  by  C.  W.  Waggoner;  (3)  a 
group   of    four    phosphorescent   sul- 
phides    furnished    by    Leppin    and  QL 
Masche,  and  prepared  by  the  method 
of  Lenard  and  Klatt. 

The  phosphoroscope  described  on 
page  109  was  used  in  connection  with 
a  special  form  of  photometer,  the 
arrangement  of  which  is  indicated  in 
Fig.  128. 

In  the  diagram  D  is  the  revolving 
disk  of  the  phosphoroscope,  5  the 
spark  gap,  C  the  phosphorescent  sub- 
stance, M  the  revolving  mirror.  The 
photometer  was  made  of  two  brass 

tubes  Z  and  X  set  at  right  angles,  Fig.  128. 

with  a  mirror  A  mounted  at  45°  at 

the  point  of  intersection  of  their  axes.  The  mirror  was  a  piece  of  microscopic 
slide  glass  cut  in  the  form  of  an  oval  with  a  hole  in  the  center  and  then 
silvered.  This,  when  viewed  from  E,  gave  a  contrast  field  with  a  patch 
of  phosphorescent  light  in  the  middle  surrounded  by  a  disk  of  light 

'Carl  A.  Zeller,  Physical  Review,  xxxr,  p.  367. 


122 


STUDIES   IN   LUMINESCENCE. 


reflected  from  the  comparison  lamp  L.  The  color  match  was  obtained 
by  using  screens  consisting  of  films  stained  with  various  aniline  dyes  on 
glass.  The  lamp  L  was  mounted  on  a  photometer  bar  and  could  be  moved 
through  a  considerable  range  of  distances. 

THE   ANILINE   DYES. 

The  aniline  dyes  were  dissolved  in  either  zapon,  gelatin,  water  glass, 
or  collodion,  and  flowed  on  glass.  The  glass  proved  to  be  too  phos- 
phorescent for  its  use;  even  oxidized  brass  emitted  some  light.  Finally 
black  broad-cloth  was  used  as  a  background,  and  the  samples  were  tested 

in  powdered  form,  the  phospho- 
roscope  running  at22oo  R.P.M. 
From  a  lot  of  fifty  samples  ob- 
tained from  Heller  and  Merz, 
the  following  show  both  fluores- 
cence (F)  and  phosphorescence 
(P)  in  slight  degree :  Naphthyl 
carmine,  white ;  F  blue,  P  very 
slight.  Naphthyl  sodium  di- 
sulphonate,  gray;  F  blue,  P 
slight.  Phthalic  anhydride, 
white;  F  light  blue,  P  slight. 
Naphthol  sodium  disulphonate, 
gray;  F  blue,  P  slight.  Naphthol 
sodium  monsulphonate,  gray; 
F  blue,  P  slight.  Naphthol  so- 
dium sulphonate,  gray;  F  blue, 
P  trace.  Beta-naphthol,  white ; 
F  light  blue,  P  trace.  Tetra- 

.03      .04      .05      .06  chlor  phthalic   anhydride, 

Seconds  white ;  F  blue,  P  slight.     Alpha 

naphthylamine ;  F  violet,  P 
slight.  Nitro-naphthylamine, 
light  yellow;  F  green,  P  slight. 
Sulphonate  of  soda,  reddish;  F  blue,  P  slight.  Several  of  these  show 
marked  fluorescence  when  dissolved.  Eosin,  fluorescein,  and  rhodamin,  in 
powdered  form,  show  no  trace  of  phosphorescence. 

THE   MANGANESE  CHLORIDE   GROUP    NaCl-MnCl2. 

The  substances  had  been  prepared  by  Dr.  Waggoner  by  mixing  solutions 
of  MgCl2  and  NaCl  in  known  proportions  and  evaporating  to  dryness. 
The  percentage  of  MnC^  varied  in  different  cases  from  o.oi  per  cent  to 
2.0  per  cent. 

The  samples  were  placed  on  brown  cardboard  with  zapon  varnish.  The 
phosphoroscope  ran  at  600  revolutions  per  minute,  or  one  revolution  in 
o.i  second.  Time  of  excitation  0.025  second.  The  zero  point  was  deter- 
mined by  readings  taken  at  the  point  where  reflected  white  light  just 
disappeared.  In  making  a  reading  the  mirror  was  turned  just  far  enough 
to  shut  off  the  direct  reflected  light.  The  standard  was  then  moved  until 


Fig.  129. 

Manganese-chloride  group,  ranging  from  o.oi  to  20 
percentage  concentration. 


STUDIES   OF   PHOSPHORESCENCE   OF   SHORT   DURATION. 


123 


a  definite  match  was  obtained.  The  rest  of  the  readings  were  made  by 
moving  the  standard  lamp  one  division  and  turning  the  mirror  away  from 
the  zero  point.  The  dimness  of  the  phosphorescent  light  and  the  limit  of 
the  machine  allowed  but  seven  or  eight  points  at  most.  The  data  plotted 
are  the  averages  of  three  or  four  readings.  The  phosphorescence  of  this 
group  is  a  pinkish  red.  In  Fig.  129  the  curves  are  plotted  with  time  of 
decay  as  abscissas  and  i/\/I  as  ordinates,  i/\/I  being  proportional  to  the 
distance  of  the  standard  lamp  from  the  photometer  screen.  The  lower  and 
upper  parts  of  the  curves  are  straight  lines,  becoming  concave  downward 
as  they  approach  each  other.  Several  of  the  curves  happen  to  have  points 
taken  at  the  bend  and  would  indicate  two  straight  lines  meeting.  All  of 
the  curves  plotted  with  7~*  as  abscissas  show  the  upper  and  lower  parts  as 
straight  lines,  gradually  merging  into  one  another. 

The  curves  are  in  fact  of  exactly  the  same  type  as  the  decay  curves 
obtained  with  Sidot  blende.1  A  glance  at  the  curves  in  Fig.  129  shows  a 
marked  tendency  toward  parallelism  in  both  parts.  According  to  the 
assumption  of  Wiedemann  and  Schmidt,  that  the  light  is  emitted  during  the 


IE3A56789IO 
Percentage  of  concentration 

Fig.  130. 

Showing  relation  between  intensity  and  percentage  concentration. 


.02  .04 

Seconds 
Fig.  131. 

Decay  curves  for  three  cadmium 
compounds.  The  color  of  phos- 
phorescence is  green  in  each  case. 
Time  of  excitation  =  0.025  *ec. 

A ,  CdSO«  +  MnSO4  -  phosphores- 
cence; B,  CdSO4  +  MnCl ;  C, 
CdSOi  +MnCl  —different  percent- 


recombination  of  the  products  of  the  dissociation  produced  during  excita- 
tion, this  would  indicate  the  same  coefficient  of  recombination  of  the  ions  for 
the  lower  parts  of  these  curves,  and  also  the  same,  but  slower  rate  in  the  up- 
per parts.  In  this  group  the  compounds  that  have  the  greatest  initial  inten- 
sity have  the  longest  period  of  decay,  although  there  seems  to  be  no  relation 
between  the  initial  intensity  and  the  time  of  decay.  The  cadmium  com- 
pounds (Fig.  131)  show  just  the  opposite  relation.  The  different  slant  in 
the  two  parts  of  the  curves  indicates  a  different  coefficient  of  recombination 
and  a  different  rate  of  decay  for  the  two  parts.  The  coefficient  and  the  de- 
cay are  smaller  in  the  second  part,  showing  a  much  slower  process  in  the 
giving  back  of  the  stored  energy. 

The  greatest  intensity  was  shown  by  the  compound  of  0.8  per  cent  con- 
centration.    The  relation  between  percentage  concentration  and   initial 

'See  Chapter  V. 


124 


STUDIES   IN   LUMINESCENCE. 


intensity  is  shown  in  Fig.  130  by  plotting  percentage  concentrations  as 
abscissas  and  initial  intensities  as  ordinates.  The  initial  intensities  were 
obtained  by  extending  the  straight  line  of  the  first  part  of  the  curves  in 
Fig.  129  to  the  y  axis  and  reading  the  y  intercept  from  the  scale. 

THE    CADMIUM   GROUP. 

The  phosphorescence  of  the  cadmium  compounds  is  yellow-green.  Fig. 
131  shows  the  decay  curves  for  three  different  compounds.  These  are 
characteristic  decay  curves,  and  follow  the  general  form.  The  compounds 
CdSO4+NaCO3,  CdSO4+NaClO3,  CdSO4+NaCl,  CdSO4+NaNO3,  and 
CdSO4+NaBr  were  too  dim  to  measure,  but  when  excited  may  be  seen 
in  a  dark  room  for  several  minutes.  In  this  group  the  specimens  that  have 
the  most  intense  phosphorescence  have  the  most  rapid  decay. 

SUBSTANCES   OF   SLOW  DECAY. 

A  number  of  phosphorescent  sulphides,  made  by  the  method  of  Lenard 
and  Klatt,1  were  obtained  from  Leppin  and  Masche,  Berlin.  They  consist 

of  a  sulphide,  an  active  metal,  and  a  flux. 
Practically  all  long-time  and  short-time 
decay  curves  have  shown  two  distinct 
processes,  merging  into  one.  By  this 
method  it  was  possible,  using  long-time 
specimens,  to  study  the  first  part  of  the 
first  process,  for  the  very  first  interval  of 
decay.  The  specimens  were  excited  for 
0.008  second  and  the  first  readings  made 
0.0014  second  after  excitation.  The 
curves  shown  in  Fig.  132,  plotted  with 
time  as  abscissas  and  7~-  as  ordinates, 
are  in  all  cases  straight  throughout  the 
range  studied.  These  results,  which  are 
in  agreement  with  the  results  of  Wag- 
goner for  Sidot  blende,  make  it  appear 
quite  improbable  that  the  behavior  of 
2  Seconds  tne  phosphorescent  sulphides  during  the 

Fig.  132.  early  stages  of  decay  can  be  represented 

A-  Sr-Bi-Na  phosphorescence,  green;  B.  Sr-  by  an   exponential  law,  as  has   recently 

Zn-F  phosphorescence,  green;  C.  Ca-Bi-Na    ,  f. 

phosphorescence,  violet ;  D.  Ba-Cu-Li  phos-  been  predicted  by  Lenard.       It  should 

phorescence,  orange-red.  ,  -^1^1  j.1  j.t          i  r 

be  pointed  out,  however,  that  the  slant  ot 

the  straight  lines  in  Fig.  132  is  such  as  to  indicate  an  initial  rate  of  decay 
far  greater  than  that  corresponding  to  what  is  usually  called  the  "first 
process,"  or  "Momentan-prozess,"  in  the  decay  of  substances  of  this 
class.  It  is  possible  that  under  the  conditions  of  these  experiments  the 
decay  curves  of  the  phosphorescent  sulphides  consist  of  three  portions, 
each  of  which  is  linear,  or  nearly  so,  when  7~*  is  plotted  against  time. 

'See  Kayser,  Handbuch  der  Spectroscopie,  vol.  4,  p.  750.     2Lenard   Annalen  der  Physik,  xxi.  p.  641,  1910. 


CHAPTER  VIII. 
PHOTOGRAPHIC  STUDIES  OF  LUMINESCENCE. 

Dr.  C.  A.  Pierce,  whose  investigations  of  thermo-luminescence  have 
been  described  in  Chapter  VI,  has  attacked  by  a  photographic  method 
some  of  the  problems  which  arise  in  the  study  of  the  relations  of  fluorescence 
and  phosphorescence  and  in  the  consideration  of  the  form  of  the  curve  of 
decay.  An  account  of  his  method  and  a  summary  of  his  results,  which 
have  an  important  bearing  upon  these  questions,  are  given  in  the  present 
chapter.1 

THE  DISTRIBUTION  OF  ENERGY  IN  THE  FLUORESCENCE  SPECTRUM  AND  THE 
PHOSPHORESCENCE  SPECTRUM  OF  SI  DOT  BLENDE. 

The  method  used  in  the  study  of  the  distribution  of  energy  in  fluorescence 
spectra  consisted  of  photographing  on  the  same  plate  the  spectrum  of  the 
excited  substance2  and  four  spectra  of  an  acetylene  flame.  Different  in- 
tensities of  the  light  from  this  flame  were  used  for  the  four  spectra,  but  the 
times  of  exposure  of  all  four  and  of  the  fluorescence  spectrum  were  the  same. 
Knowing  the  distribution  of  energy  in  the  acetylene  flame,3  the  distribution 
in  the  fluorescence  light  was  obtained  by  photometric  comparisons  of  the 
spectra  on  the  plate. 

During  the  set  of  experiments  the  fluorescent  powder  was  contained  in  a 
square  dish  made  of  platinum  foil.  The  foil  was  connected  to  and  sup- 
ported by  copper  leads,  so  that  the  temperature  of  the  powder  could  be  raised 
and  controlled  by  passing  electric  current  through  the  foil.  The  temper- 
ature was  measured,  if  different  from  room  temperature,  by  means  of  a 
copper-constantan  thermo-couple  placed  in  the  midst  of  the  powder.  The 
blue  lines  of  a  mercury-arc  lamp  were  used  to  excite  the  powder.  The 
various  intensities  of  acetylene  light  were  obtained  by  screening  off  all  but 
the  center  of  an  acetylene  flame  and  moving  the  flame  with  the  screen  to 
different  distances  from  the  slab  of  magnesia  which  reflected  the  light  into 
the  slit  of  the  spectrum  camera.  The  camera  consisted  of  a  direct-vision 
spectroscope  set  so  that  the  spectrum  could  be  focused  on  a  sensitive  plate 
held  in  a  plate  holder. 

The  plates  were  handled  and  developed  in  complete  darkness.  The 
developer  was  freshly  mixed  from  a  stock  solution  and  of  a  standard 
strength,  and  the  length  of  development  was  timed,  being  for  most  of  the 
plates  8  minutes.  The  temperature  was  brought  to  20°  at  the  beginning 
of  development  and  the  plates  were  rocked  mechanically  during  the  de- 
velopment. The  plates  were  fixed,  at  first  for  30  minutes,  after  washing 
in  three  separate  waters,  and  the  temperature  of  the  fixing  bath  was  brought 


'See  C.  A.  Pierce,  Physical  Review,  xxx,  p.  663;  xxxii,  p. 115. 

2The  substance  employed  was  the  "  Emanations-pulver  "  used  in  the  experiments  described  in  Chapter  IV. 
3On  the  distribution  of  energy  in  the  visible  spectrum,  by  E.  L.  Nichols;  Physical  Review,  xxr,  p.  147, 
Sept.  1905. 

125 


126 


STUDIES   IN   LUMINESCENCE. 


to  20°  at  the  beginning  of  the  fixing.  Later  not  so  much  care  was  used  in 
the  fixing  because  it  was  found  to  be  inadvisable  to  intercompare  spectra 
on  different  plates.  After  fixing,  the  plates  were  washed  in  running  water 
for  about  30  minutes,  then  thoroughly  rubbed  by  hand  and  dried.  Each 
plate  was  dried  in  such  a  position  that  the  last  portion  to  drain  was  a  part 
upon  which  no  measurements  were  to  be  made. 

The  distribution  of  denseness  in  any  spectrum  on  a  plate  was  obtained 
by  pushing  the  plate  past  a  brightly  illuminated  slit  placed  squarely  across 
the  spectrum,  and  measuring  the  transmitted  light  by  means  of  a  Lummer- 
Brodhun  photometer.  The  source  of  light  to  illuminate  the  slit  and  the 
standard  light  consisted  of  two  carbon-filament  electric  lamps  with  frosted 
bulbs.  The  light  back  of  the  plate  was  concentrated  on  the  slit  by  a 
reflector.  The  current  supplied  to  the  lamps  was  held  practically  constant. 
It  was  found  by  actual  test  that  a  change  of  5  volts  was  necessary  to  vary 
the  relative  intensities  of  the  lamps  enough  to  affect  the  settings  of  the 
photometer  appreciably.  The  lamps  were  run  at  6  per  cent  above  normal 
voltage  to  increase  the  candlepower  and  were  lighted  only  long  enough  to 
make  the  measurements. 


.66 


The  wave-length  measurements  were  made  by  calibrating  in  wave- 
lengths the  screw  which  pushed  the  plate  past  the  slit  on  the  photometer 
bar.  A  zero  wave-length  was  obtained  on  the  plate  by  photographing  the 
three  blue  lines  of  the  mercury  arc  superimposed  on  the  different  spectra. 
These  lines  were  in  an  entirely  different  region  from  that  occupied  by  the 
spectrum  of  the  fluorescence  light.  The  calibration  of  the  screw  was  made 
by  photographing  the  entire  visible  spectrum  of  the  mercury  arc  with  the 
spectrum  camera  and  setting  one  line  after  the  other  directly  in  front  of  the 
slit  on  the  photometer  bar.  This  slit  was  always  opened  just  as  wide  as  the 
width  of  the  lines  in  the  spectra  and  was  placed  parallel  to  these  lines.  The 
standard  lamp  was  screened  off  by  means  of  a  variable  slit,  so  that  the  pho- 
tometer could  be  used  on  whatever  part  of  the  bar  was  desired. 

Fig-  133  shows  the  curves  obtained  by  plotting  the  light  transmitted 
through  the  spectra  on  one  of  the  plates  which  had  been  exposed  as  de- 
scribed above.  Each  exposure  was  60  minutes  long  and  was  made  with  the 


PHOTOGRAPHIC   STUDIES   OF   LUMINESCENCE. 


127 


slit  opened  40  units.  Curves  1,2,3,  and  4  represent  the  spectra  of  acetylene 
light  at  intensities  32,  16,  8,  and  4  respectively.  Curve  7  represents  the 
spectrum  of  the  fluorescence  light.  Curve  6  represents  the  distribution 
of  energy  in  the  acetylene  flame  and  is  assumed  to  correspond  to  curve  3. 
Then  the  energy  curve  corresponding  to  curve  2  will  have  ordinates  equal 
to  1 6/8  times  the  corresponding  ordinates  of  curve  6,  etc.  Wherever  curve 
7  crosses  curve  3,  the  energy  for  that  wave-length  is  represented  by  the 
ordinate  on  curve  6  for  that  wave-length.  Wherever  curve  7  crosses  curve 
2,  the  energy  for  that  wave-length  is  16/8  times  the  ordinate  of  curve  6  for 
that  wave-length,  etc. 

Since  the  curves  intersect  at  only  a  few  points,  it  was  necessary  to  get  other 
points  on  the  desired  energy  curve  by  interpolation.  This  was  done  sys- 
tematically and  as  follows:  Suppose  wave-length  m  is  under  consideration. 
A  curve  is  plotted  with  the  intensities  of  acetylene,  4,  8,  16,  and  32,  as 
abscissas  and  the  intersections  of  the  vertical  at  m  with  the  curves  i,  2,  3, 


and  4  as  ordinates.  From  this  curve  is  picked  off  the  intensity  of  acety- 
lene that  would  have  coincided  with  curve  7  at  wave-length  m.  Fig.  134 
shows  six  curves  drawn  in  connection  with  Fig.  133.  Curves  i,  2,  3,  4,  5, 
and  6  correspond  to  wave-lengths  ^  =  0.50,  0.51,  0.52,  0.53,  0.54,  and  0.55 
respectively.  The  circle  on  each  curve  shows  the  point  picked  off.  On 
curve  6,  Fig.  134,  23.5  is  the  abscissa  corresponding  to  2.8,  the  ordinate  on 
curve  7,  Fig.  133,  at  /z  =  0.55.  Hence  the  ordinate  at  n  =0.55  on  curve  5, 
Fig-  i33»  which  shows  the  distribution  of  energy  of  the  fluorescence  light  is 

23.5 
equal   to  — —  times  21.3,  the  ordinate  on  curve  6,  Fig.  133,  at  ^  =  0.55. 

Curve  5,  Fig.  133,  shows  the  energy  curve  of  fluorescence  as  computed 
from  the  other  curves  in  Fig.  133.  The  curve  has  a  well-defined  maximum 
at  /x  =  0.55  and  a  minimum  at  /x  =  o.6o.  The  shape  of  the  curve  for  wave- 
lengths greater  than  /x  =  o.6o  is  uncertain. 

Fig.  135  shows  another  set  of  curves  corresponding  to  those  in  Fig.  133. 
The  length  of  exposure  was  30  minutes  for  each  spectrum  with  a  slit  width 


128 


STUDIES   IN   LUMINESCENCE. 


of  20  units.  The  intensities  of  the  acetylene  flame  were  32,  20,  12,  and  6 
for  curves  i,  2,  3,  and  4  respectively.  Curve  6  is  assumed  to  represent  the 
energy  distribution  in  curve  4.  Curve  5,  which  shows  the  distribution  of 
energy  in  the  fluorescence  light,  is  similar  to  the  corresponding  curve  5  in 
Fig.  133-  This  shows  that  the  curve  is  not  materially  influenced  by  two 
factors,  length  of  exposure  and  width  of  slit  in  the  spectroscope. 

The  Sidot  blende  was  excited  by  three  blue  lines  of  the  mercury-arc 
spectrum.  While  these  lines  are  in  a  different  region  of  the  spectrum  from 
that  occupied  by  the  fluorescence  band,  yet  there  was  considerable  halation 
shown  on  the  plates  about  the  three  lines  and  it  was  thought  that  this  might 
extend  far  enough  to  change  the  shape  of  the  curves  in  the  region  of  smaller 
wave-lengths. 

To  test  this  matter  a  slab  of  magnesium  carbonate  was  substituted  for  the 
Sidot  blende  and  was  illuminated  in  the  same  manner  by  the  blue  and 
violet  lines  of  the  mercury  arc. 

A  plate  was  exposed  to  the  light  reflected  from  this  slab  and  also,  as  in 
the  previous  experiments,  to  four  different  intensities  of  acetylene  light. 
Curves  showing  the  amount  of  transmitted  light  are  given  in  Fig.  136. 


4-2  .46  .50  .54  .58  .62  .66 

Fig.  136. 

In  this  as  in  the  previous  figures  curves  i,  2,3,  and  4  are  for  the  acetylene 
light.  Curve  5  indicates  the  transmission  of  that  part  of  the  plate  exposed 
to  the  light  of  the  mercury  arc  reflected  by  the  magnesium  carbonate. 
Since  there  is  no  indication  of  any  effect  it  appears  that  halation  from  the 
mercury  lines  is  inappreciable  for  wave-lengths  longer  than  0.46  ju  and  that 
no  error  from  this  source  occurs  in  the  curves  marked  5  in  Figs.  133  and  135. 

Experiments  were  also  made  with  very  intense  negatives  in  an  endeavor 
to  obtain  the  shape  of  the  energy  curves  for  wave-lengths  greater  than  0.60^1, 
but  on  account  of  an  apparent  tendency  to  reversal  in  such  negatives  it  was 
not  found  possible  by  this  method  to  extend  observations  to  these  longer 
waves. 

PHOSPHORESCENCE  AT  ROOM  TEMPERATURE. 

In  order  to  obtain  the  distribution  of  energy  in  the  phosphorescence 
light,  it  was  necessary  to  add  to  the  apparatus  already  described  a  shutter 
which  would  close  the  spectrum  camera  and  excite  the  powder;  then  shut 
off  the  exciting  light  and  open  the  camera.  The  shutter  was  operated  by 
a  constant-speed  motor.  Levers,  springs,  and  triggers  were  so  arranged 


PHOTOGRAPHIC   STUDIES  OF   LUMINESCENCE. 


129 


that  the  shutter  operated  in  a  small  part  of  a  second.  The  length  of 
excitation  and  decay  could  be  increased  or  decreased  together  by  changing 
the  speed  of  the  motor,  while  either  the  length  of  excitation  or  the  length  of 
decay  could  be  varied  alone  by  rearrangement  of  parts  of  the  controlling 
devices.  The  total  time  of  exposure  of  the  photographic  plate  to  the  phos- 
phorescence light  was  made  equal  to  the  length  of  exposure  of  the  acetylene 
spectra. 

With  the  apparatus  described  above,  the  photographic  negative  of 
phosphorescence  was  obtained  by  light  which  varied  in  intensity,  for  the 
phosphorescence  decayed  after  the  exciting  light  was  closed  off  in  the 
manner  characteristic  of  phosphorescent  powders.  However,  if  an  energy 
curve  is  obtained  similar  to  that  for  fluorescence,  it  seems  reasonably  certain 
not  only  that  the  energy  curve  does  not  change  with  decay,  but  also  that  it 
does  not  change  when  the  exciting  light  is  shut  off. 


44- 


Fig.  137  shows  the  energy  distribution  in  the  phosphorescence  light  of 
Sidot  blende.  The  powder  was  excited  each  time  for  8.75  seconds  and 
allowed  to  decay  10.33  seconds.  The  powder  was  not  exposed  to  infra-red 
before  each  excitation.  The  effective  length  of  exposure  of  the  photo- 
graphic plate  was  60  minutes.  Curves  i,  2,  3,  and  4  show  the  transmitted 
light  of  the  photographic  spectra  corresponding  to  intensities  of  acetylene 
equal  to  6,  4,  2,  and  r,  respectively.  Curve  5'  shows  the  transmitted  light 
of  the  phosphorescence  spectrum.  Curve  5  shows  the  energy  curve  for  the 
phosphorescence  light. 

It  is  seen  that  curve  5  is  similar  in  form  to  the  corresponding  curves  for 
fluorescence.  The  phosphorescence  spectrum,  at  least  in  the  early  stages 
of  decay,  is  thus  the  same  as  the  fluorescence  spectrum. 

DISCUSSION  OF   METHOD. 

The  chemical  change  made  in  a  photographic  plate  depends  upon  the 
wave-length  and  intensity  of  the  incident  light  and  upon  the  length  of 


130  STUDIES   IN   LUMINESCENCE. 

exposure.  If  several  spectra  are  photographed  upon  the  same  plate  and 
the  length  of  exposure  is  the  same  for  all  the  spectra,  then  the  denseness  of 
the  different  negatives  at  any  wave-length  will  depend  only  upon  the 
intensity  of  the  incident  light  at  that  wave-length,  assuming  a  small  opening 
of  the  slit  to  the  spectroscope,  uniform  development,  non-halation,  etc. 
Since  intensity  is  proportional  to  the  rate  at  which  energy  is  received  from 
the  source,  the  denseness  of  the  different  negatives  at  any  wave-length 
depends  upon  the  energy  received  from  the  source  at  that  wave-length. 
By  making  several  negatives  of  one  source  of  light  of  known  energy  dis- 
tribution at  different  distances  from  the  spectroscope,  curves  can  be  drawn 
showing  the  relation  between  energy  received  at  any  wave-length  and  dense- 
ness  of  negative.  If  a  spectrum  of  another  light  of  unknown  energy  dis- 
tribution is  photographed  on  the  same  plate,  its  energy  distribution  can  be 
obtained  by  comparison  of  the  denseness  of  its  negative  with  the  calibrated 
densities.  The  densities  can  be  compared  by  means  of  the  light  transmitted 
through  the  negatives  at  different  wave-lengths. 

If  the  slit  opening  in  the  spectroscope  is  not  very  small,  the  denseness  of 
the  negative  at  any  wave-length  will  not  depend  alone  upon  the  energy 
received  at  that  wave-length,  but  will  also  depend  upon  the  energy  received 
at  wave-lengths  differing  but  little  from  the  wave-length  in  question. 
Consequently  a  small  dimple  in  a  curve  might  be  obliterated  by  a  wide  slit 
opening.  The  effect  of  slit  opening  can  be  tested  by  using  different  openings, 
finally  using  the  one  which  experience  shows  to  be  best. 

The  effect  of  halation  is  more  troublesome  than  slit  opening,  and  was 
present  more  or  less  in  all  of  the  negatives.  The  effect  of  halation  is  to 
broaden  the  band  of  energy  distribution,  but  not  to  change  the  position 
of  the  maximum  point  of  the  band.  It  is  believed  that  halation  in  the 
present  experiments  was  not  of  sufficient  effect  to  cause  any  serious  error  in 
the  curves. 

The  plates  were  developed  immediately  after  the  exposures,  which  were 
made  one  directly  after  the  other,  so  that  any  error  due  to  continued  chemi- 
cal action  after  exposure  in  the  film  was  eliminated  as  far  as  possible. 

The  photometric  measurements  were  difficult  to  make  because  of  the 
large  differences  in  the  amount  of  light  transmitted  in  different  parts  of  the 
negatives.  At  the  beginning  of  the  experiments,  four  settings  of  the  pho- 
tometer were  made  for  each  point  measured .  Later  two  settings  were  found 
to  be  sufficient,  one  approaching  uniformity  of  illumination  from  each 
direction.  The  average  of  the  two  values  calculated  from  the  two  settings 
was  taken  as  the  true  ratio  of  standard  and  transmitted  lights.  Remeasure- 
ment  of  a  set  of  curves  never  proved  any  exceptional  accuracy,  but  always 
gave  the  same  type  of  energy  curve  with  the  maximum  at  nearly  the  same 
wave-length.  Consequently  the  measurements  can  be  said  to  be  substan- 
tially correct. 

These  experiments  show  that  the  energy  curve  of  the  fluorescence  light  of 
Sidot  blende  consists  of  a  band  extending  from  about  ^  =  0.46  to  ju  =  o.6o, 
having  a  maximum  at  /*  =0.55.  There  may  be  another  band  situated  in  the 
region  of  longer  wave-lengths.  Furthermore,  the  energy  distribution  in  the 
fluorescence  light  and  the  phosphorescence  light  immediately  after  excita- 
tion is  the  same. 


PHOTOGRAPHIC   STUDIES   OF   LUMINESCENCE. 


THE  PHOSPHORESCENCE  SPECTRUM  DURING  DECAY  AND  THE  QUESTION  OF 
TWO  OVERLAPPING  BANDS. 

In  Chapters  IV,  V,  VI,  and  VII  of  this  memoir,  the  characteristic  form 
of  the  curve  of  decay  of  phosphorescence  for  various  substances  and  under 
a  variety  of  conditions  has  been  given  and  it  has  been  shown  to  consist  of 
two  straight  lines  of  different  slopes  which  gradually  merge  into  one  another. 
In  Chapter  VI  it  was  shown,  moreover,  that  Becquerel's1  expression  for 
the  decay  could  be  made  to  fit  the  actual  curves  by  assuming  the  existence 
of  two  overlapping  bands  having  very  different  rates  of  decay.2 

In  Chapter  IV  measurements  on  the  phosphorescence  of  Sidot  blende 
were  described  and  it  was  shown  that  in  the  case  of  Sidot  blende  the  spec- 
trum did  not  change  its  form  during  the  first  few  seconds  of  decay.3  A 
similar  result  was  obtained  by  Waggoner4  (see  Chapter  VII)  for  several 
other  substances  exhibiting  phosphorescence  of  short  duration. 

While  the  presence  of  overlapping  bands  of  differing  duration  would 
necessarily  cause  a  gradual  change  in  the  distribution  of  intensities  in  the 
phosphorescence  spectrum  as  decay  progressed,  these  measurements  were 
not  decisive,  as  against  the  hypothesis  of  the  existence  of  such  bands.  It 
was  found  in  the  case  of  the  Sidot  blende  that  the  "first  process"  of  decay 
lasted  about  10  seconds,  whereas  the  spectrophotometric  measurements 
covered  only  3  or  4  seconds.  Dr.  Waggoner's  determinations  likewise  apply 
chiefly  to  the  earlier  stages  of  decay. 

The  experiments  now  to  be  described  were  undertaken  to  provide  further 
data  bearing  upon  the  two-band  theory  of  phosphorescence  decay.  The 
two  rectilinear  parts  of  the  typical  decay  curve  differ  widely  in  slope. 
Hence,  on  the  assumption  of  two  bands,  most  of  the  light  before  the  bend 
in  the  curve  is  due  to  the  band  corresponding  to  the  steeper  straight  line 
(see  Fig.  79,  Chapter  VI) .  At  the  bend  the  light  is  due  more  or  less  equally 
to  both  bands;  and  after  the  bend  the  light  is  due  mostly  to  the  second 
band.  With  a  method  available  for  studying  the  light  distribution  before 
and  after  the  bend,  the  two-band  explanation  would  be  proved  or  disproved, 
depending  on  whether  a  change  was  or  was  not  found  in  the  distribution  of 
the  light. 

As  was  shown  in  the  first  part  of  this  chapter,  the  fluorescence  spectrum 
of  the  substance  used  consisted  of  one  prominent,  symmetrical,  smooth- 
sided  band  with  a  maximum  at  about  ^  =  0.55  (see  Fig.  133).  The  band 
extends,  approximately,  from  /i  =  0.46  to  n  =  0.60.  Furthermore,  the  energy 
distribution  immediately  after  excitation  (Fig.  137)  is  the  same  as  in  the 
fluorescence  spectrum. 

The  present  work  is  concerned  with  the  light  distribution  before  and 
after  the  bend  in  the  decay  curve. 

>H.  Becquerel.  Comptes  Rendus,  113,  p.  618,  1891. 
;See  Chapter  VI;  also  C.  A.  Pierce,  Physical  Review,  xxvi,  p.  312. 
'See  also  Nichols  and  Merritt,  Physical  Review,  xxi,  p.  247. 
•C.  W.  Waggoner,  Physical  Review,  xxvn,  p.  220. 


132 


STUDIES    IN    LUMINESCENCE. 


EXPERIMENTAL. 

The  method  employed  to  photograph  the  decaying  band  of  phospho- 
rescence was  based  on  the  assumption  that  the  conditions  and  phenomena 
could  be  reproduced  indefinitely,  a  fact  already  established  for  the  substance 
under  investigation.  The  phosphorescent  powder,  except  in  the  case  of  Fig. 
140,  which  gives  the  results  obtained  with  Balmain's  paint,  was  zinc  sul- 
phide, i.  e.,  the  sample  of  "  Emanations-pulver  "  used  in  the  investigations 
described  in  Chapter  VI. 

The  apparatus  was  that  described  on  pages  130  and  131,  but  electri- 
cally operated  shutters  were  used  to  excite  the  powder,  to  expose  the  plate, 
and  to  kill  off  the  remaining  phosphorescence  with  infra  radiations  before 
repeating  the  excitation.  With  this  apparatus  the  decay  curve  could  be 
photographed  between  any  two  points,  time  and  time  again,  until  an  im- 
pression had  been  made  on  the  photographic  plate.  Since,  at  best,  the 
necessary  exposure  was  very  long,  varying  from  a  few  hours  to  a  much 
longer  time,  no  attempt  wras  made  to  deduce  the  energy  distribution,  but 

the  photographic  spectra 
were  compared  with  each 
other,  the  principal  weight 
being  attached  to  the  posi- 
tion of  the  maximum  of 
the  band.  Care  was  taken 
to  obtain  negatives  of 
about  the  same  average 
density,  and  of  not  too 
great  density,  so  that  no 
complications  could  result 


.4.6 


.50 


.54- 


Fig.  138. 


from  widely  different  or 
complete  chemical  change 
of  the  films  at  any  wave- 
length. With  these  precautions,  negatives  were  obtained  which  showed 
definite  maxima,  and  the  results  could  be  repeated  as  many  times  as  desired. 
The  distribution  of  denseness  on  the  photographic  film  was  measured, 
at  first,  with  a  photometer,  as  in  the  previous  experiments ;  but  this  method 
was  soon  abandoned  because  of  the  eye  strain  induced  by  comparing  very 
faint  fields  of  light.  The  apparatus  which  was  substituted  consisted  of  a 
brightly  illuminated  slit  placed  before  a  very  sensitive  system  of  thermo- 
couples. The  photographic  plate  was  pushed  past  the  slit  by  means  of  a 
screw  calibrated  in  wave-lengths.  Since  the  deflection  of  the  galvanometer, 
which  measured  the  current  from  the  thermo-couples,  depended  upon  the 
length  of  time  that  light  was  allowed  to  pass  through  the  slit,  a  pendulum 
was  made  to  light  and  extinguish  the  electric  lamp  which  illuminated  the 
slit,  at  predetermined  intervals.  With  this  apparatus  results  were  obtained 
which  were  consistent  with  those  obtained  with  the  photometer,  and  the 
results  could  be  repeated. 

Fig.  138  shows  the  data  obtained  by  measuring  a  film  with  the  pho- 
tometer (curve  A}  and  with  the  thermo-couple  (curve  B}.     In  this  and  in  all 


PHOTOGRAPHIC   STUPIES   OF   LUMINESCENCE. 


subsequent  figures  ordinates  represent  intensities  of  transmitted  light. 
These  two  curves  are  consistent  as  regards  general  shape  and  the  position 
of  the  points  of  minimum  transmission.  Since  intensities  of  transmitted 
light  are  plotted,  the  minima  of  the  curves  correspond  to  the  maxima  of 
intensities  of  the  spectra.  Inspection  will  show  that  curve  B  is  not  the 
exact  duplicate  of  curve  A ;  the  ratios  of  the  ordinates  at  long  wave-lengths 
are  not  the  same  as  at  shorter  wave-lengths.  This  lack  of  similarity  did 
not  exist  in  many  of  the  comparisons  and  was,  quite  likely,  due  either  to 
eye  fatigue  or  else  to  the  gradual  warming  up  of  the  plate  under  successive 
exposures  to  the  light  which  illuminated  the  slit. 

Fig.  139  shows  three  curves  corresponding  to  the  decay  of  phosphores- 
cence. In  curve  A,  the  Sidot  blende  was  excited  for  9.75  seconds  and  the 
plate  was  exposed  for  8.25  seconds  immediately  after  excitation.  The 
powder  was  exposed  for  about  i  minute  to  infra-red  rays  and  the  process 
was  repeated  over  and  over  until  a  sufficient  exposure  had  been  secured. 


BX 


•  SO 


.54  .58 

Fig-  139- 


^ 

r 

^^ 

o  

—  o  — 

\ 

\B 

/ 

^ 

A\ 

^ 

^^ 

s 

/ 

i 

V 

w 

X 

.38 


.50/< 


Fig.  140. 


In  curve  B  the  powder  was  excited  for  21  seconds,  allowed  to  decay  for  14 
seconds,  then  the  plate  was  exposed  for  15  seconds.  In  curve  B  the  #-axis 
is  raised  so  that  curve  B  does  not  intersect  curve  A .  In  curve  C  the  exci- 
tation was  5.5  minutes,  the  decay  1.5  minutes,  and  the  plate  was  then 
exposed  for  i  minute.  The  total  time  of  exposure  to  produce  the  negative 
from  which  curve  C  was  made  was  72  hours.  The  negative  was  faint  and 
the  film  was  somewhat  fogged,  but  remeasurements  on  the  film  always 
gave  approximately  the  same  curve.  In  fact,  curve  C  is  the  average  of  sev- 
eral remeasurements.  The  minima  of  these  curves  occur  at  wave-length 
M  =  0.555  ±0.003. 

The  bend  in  the  curve  of  decay  of  the  phosphorescence  of  Sidot  blende 
occurs  between  10  and  20  seconds  after  the  end  of  excitation  and  is  more 
pronounced  the  longer  the  excitation.  Hence  curve  A  in  Fig.  139  is  due 
chiefly  to  the  light  corresponding  to  the  decay  before  the  bend,  while  curve 
B  corresponds  to  conditions  near  the  bend,  and  curve  C  corresponds  to 
conditions  far  beyond  the  bend.  Or,  in  terms  of  the  two-band  theory, 
curve  A  corresponds  mainly  to  band  i ;  curve  B,  to  bands  i  and  2 ;  and  curve 
C  to  band  2  almost  entirely.  This  set  of  curves  shows  no  change  in  the 
maxima  of  the  spectra,  and  indeed  no  set  was  obtained  which  showed  any 
appreciable  change. 


134 


STUDIES  IN  LUMINESCENCE;. 


The  powder  from  which  Balmain's  paint  is  made,  as  will  be  seen  by  refer- 
ence to  Chapter  IV,  shows  a  more  pronounced  bend  in  the  decay  curve  than 
is  the  case  with  Sidot  blende.  Two  determinations  were  therefore  made 
using  this  substance  and  showing  the  character  of  the  spectrum  before  and 
after  the  bend.  For  each  curve  (see  Fig.  140)  the  excitation  was  5  minutes. 
For  curve  A ,  the  plate  was  exposed  20  seconds  immediately  after  excitation. 
For  curve  B,  the  phosphorescence  was  allowed  to  decay  1.5  minutes,  then 
the  plate  was  exposed  for  i  minute.  No  shift  in  the  minima  of  the  curves 
can  be  detected,  while  a  decided  change  in  the  shape  of  the  curves  would 
be  expected  if  the  two-band  theory  is  correct. 

A  PHOTOGRAPHIC  TEST  OF  THE  EFFECT  OF  INFRA-RED  RAYS. 

The  effect  of  infra-red  radiations  in  suppressing  phosphorescence  and 
fluorescence  has  been  considered  at  length  in  Chapter  V,  in  which  exper- 
iments were  described  indicating  that  all  parts  of  a  fluorescence  band  are 
suppressed  in  the  same  ratio.  As  a  check  on  that  work  and  because,  on 
the  assumption  of  the  existence  of  two  bands,  one  band  would  in  all  prob- 
ability be  suppressed  more  than  the  other,  several  runs  were  made  in  which 


.46 


Fig.  141. 

the  fluorescence  spectrum  of  Sidot  blende  under  the  influence  of  infra-red 
rays  was  photographed.  These  rays  were  obtained  from  a  16  c.p.  lamp 
held  5  cm.  from  the  powder.  The  visible  rays  were  screened  off  by  means 
of  thin  rubber. 

In  Fig.  141  curve  A  corresponds  to  the  fluorescence  spectrum  without 
infra-red  excitation,  and  curve  B  with  infra-red.  The  exposure  of  the  plate 
in  the  latter  was  twice  as  long  as  in  the  former  case.  It  will  be  seen  that  the 
minimum  is  not  changed  appreciably  from  /JL  =  0.555  by  the  exposure  to  infra- 
red and  that  the  curves  agree  very  well  in  form,  except  at  short  wave-lengths. 

In  Fig.  142,  curve  A  corresponds  to  the  fluorescence  spectrum  without 
infra-red;  curve  B  to  the  same  with  infra-red;  and  curve  C  corresponds 
to  the  phosphorescence  spectrum  just  after  excitation  and  is  added  for  ease 
of  comparison.  In  each  case  the  minimum  of  the  curve  occurs  at  ju  =  0.555 
approximately. 

The  effect  of  infra-red  excitation  on  the  fluorescence  spectrum,  according 
to  all  of  the  curves  obtained,  is  to  decrease  the  intensity  of  the  band ;  but 


PHOTOGRAPHIC  STUDIES  OF  LUMINESCENCE. 


135 


there  is  no  change  in  the  distribution  of  intensities  that  can  be  detected 
by  this  method  and  no  measurable  shift  of  the  maximum.  This  result  is 
in  accordance  with  those  recorded  in  Chapter  V  and  it  does  not  confirm 
the  two-band  theory  of  decay. 


64/6 


THE  EFFECT  OF  TEMPERATURE  ON  THE  FLUORESCENCE  SPECTRUM. 

It  has  recently  been  shown1  that  remarkable  changes  in  the  apparent 
complexity  of  fluorescence  spectra  are  in  some  cases  produced  by  changes 
in  the  temperature  of  the  substance. 

To  determine  whether  such  changes 
as  could  be  detected  by  the  present 
method  take  place  in  the  fluorescence 
band  of  Sidot  blende  when  that  sub- 
stance is  excited  at  different  temper- 
atures a  series  of  photographs  were 
taken,  the  results  of  which  are  shown 
in  Fig.  143. 

This  figure  contains  four  curves 
corresponding  to  the  fluorescence  of 
vSidot  blende  at  different  temperatures. 
In  each  case  the  length  of  excitation 
was  varied  so  as  to  give,  approximately, 
negatives  of  equal  density,  hence  the 
dimming  of  the  band  at  higher  tem- 
peratures is  not  evident,  though  it 
occurred.  In  fact,  the  powder  almost 
ceased  to  exhibit  fluorescence  when  the 
temperature  was  raised  sufficiently. 

When  the  temperature  was  lowered  the  powder  would  again  show  the 
same  spectrum  as  before,  provided  a  certain  critical  temperature  had  not 
been  exceeded.  The  .T-axis  is  changed  for  each  curve,  so  that  the  points 
may  be  entirely  distinct  from  one  another.  Curves  A,  B,  C,  and  D  were 
obtained  at  temperatures  22°,  67°,  88°,  and  120°  respectively.  No  shifting 
of  the  minimum  can  be  seen. 


.50 


.54  .58 

Fig.  143- 


JoZJU 


'Nichols  and  Merritt;  Physical  Review,  xxxn,  p.  38. 


136  STUDIES   IN    LUMINESCENCE. 


DISCUSSION  OF  RESULTS. 

The  accuracy  of  individual  measurements  with  a  non-direct  method,  such 
as  the  photographic  method  described  in  this  chapter,  is  not  great.  Con- 
clusions must  be  drawn  from  the  indications  of  many  experiments,  rather 
than  from  the  too  exact  interpretation  of  a  single  determination.  A  general 
survey  of  the  curves  obtained,  of  which  only  few  examples  have  been  given, 
shows  that  the  minima,  corresponding  to  the  maxima  of  the  usual  curves  of 
distribution  previously  employed,  occurwithin  the  limits  X  =  o. 555/x  =*=  0.003. 
These  limits  were  seldom  exceeded  and  the  variations  were  not  consistent 
with  each  other.  For  some  sets  of  curves  the  limits  could  be  contracted  to 
±o.oo2  n  or  possibly  to  ^o.ooi/x.  Hence,  if  the  maximum  of  the  band 
changed  under  any  of  the  conditions,  it  must  have  been  within  the  limit 
±  0.003  M  and  probably  within  even  smaller  limits.  From  certain  considera- 
tions, such  as  the  bend  in  the  decay  curve  of  phosphorescence,  and  the 
smooth,  symmetrical  shape  of  the  whole  band,  one  would  expect,  if  the  two- 
band  theory  is  correct,  a  considerable  change  in  the  band  under  the  condi- 
tions studied.  No  changes  were  found  that  were  not  explained  by  the 
limitations  of  the  method. 

Hence  the  following  conclusions  are  drawn  from  the  work  above  : 

1 .  The  fluorescence  and  phosphorescence  bands  of  Sidot  blende  coincide 
with  each  other. 

2.  No  change  in  the  position  of  the  band  of  phosphorescence  occurs  with 
decay  in  the  case  of  either  Sidot-blende  or  Balmain's  paint. 

3.  No  change  in  the  position  of  the  band  of  fluorescence  occurs  under  the 
action  of  infra-red  rays. 

4.  No  change  in  the  position  of  the  band  of  fluorescence  occurs  with  a 
change,  between  +20°  and  +120°,  in  the  temperature  of  the  powder. 

5.  No  change  in  the  shape  of  the  band,  discernible  by  this  method,  was 
found  under  any  of  the  above  conditions. 


CHAPTER  IX. 

A  SPECTROPHOTOMETRIC  STUDY  OF  CERTAIN  CASES  OF 
KATHODO-LUMINESCENCE^ 

The  spectrophotometric  study  of  luminescence  has  thus  far  been  confined 
almost  entirely  to  cases  of  photo-luminescence.  It  is  of  interest  to  inquire 
whether  exciting  agents  other  than  light  will  give  luminescence  spectra  of 
the  same  type,  and  to  what  extent  the  laws  that  have  been  found  to  hold 
in  the  case  of  photo-luminescence  possess  a  more  general  application.  The 
experiments  on  kathodo-luminescence  described  in  the  present  chapter  are 
of  interest  chiefly  because  of  the  bearing  of  the  results  upon  these  and 
similar  questions.  During  the  course  of  the  work  preliminary  data  have 
also  been  obtained  with  regard  to  the  de- 
pendence of  kathodo-luminescence  upon 
discharge  potential  and  current  strength. 

The  form  of  the  vacuum  tube  used  is 
shown  in  Fig.  144,  the  substance  to  be 
tested  being  placed  at  S.  In  order  to  pre- 
vent disturbance  from  the  fluorescence  of 
the  glass  the  specimen  in  some  instances 
was  placed  in  a  metal  inclosure  which 
protected  the  glass  from  excitation  by  the 
kathode  rays.  The  kathode,  A",  was  a  flat  •* — 
aluminum  disk.  The  fluorescent  light  was  1,1 

reflected  directly  into  the  slit  of  the  Lummer- 
Brodhun  spectrophotometer  by  a  small  ^ 

mirror  at  the  side  of  the  tube.  An  acetylene 
flame  was  used  as  a  comparison  source. 
Current  was  furnished  by  a  motor-driven 
Holtz  machine  capable  of  giving  a  current  of 
0.6  milliampere.  A  galvanometer  placed  Fig.  144. 

in  the  circuit  next  to  the  grounded  pole  of 

the  machine  served  to  indicate  the  constancy,  or  lack  of  constancy,  of  the 
current  through  the  tube.  In  most  cases  relative  values  only  of  the  current 
have  been  recorded.  The  potential  difference  between  the  terminals  of  the 
tube  was  measured  by  a  Kelvin  electrostatic  voltmeter  and  served  as  a 
sensitive  indicator  of  changes  in  the  vacuum.2 

Considerable  difficulty  was  encountered  in  keeping  the  discharge  through 
the  tube  steady.  vSudden  changes  occurred  in  the  intensity  of  the  fluores- 

>The  results  contained  in  this  chapter  were  in  part  presented  to  the  American  Physical  Society,  at  the 
meeting  held  in  Washington,  April  24,  1908,  and  were  published  in  the  Physical  Review,  xxvm,  p.  349. 

zSince  the  absolute  values  of  the  potential  used  are  of  no  great  significance  in  the  present  work,  we  have 
plotted  voltmeter  rea'lin^s  rather  than  the  actual  values  of  the  potential  in  the  curves  contained  in  this 
chapter.  If  the  potential  in  volts  is  desired  this  may  be  found  bv  multiplying  the  voltmeter  deflection  by 
385. 

137 


138  STUDIES   IN   LUMINESCENCE. 

cence,  even  without  any  corresponding  change  in  current  or  voltage,  due 
apparently  to  some  erratic  cause  producing  either  a  deflection  of  the  kathode 
rays  or  a  change  in  the  intensity  of  these  rays.  The  most  serious  of  these 
disturbances  were  traced  to  irregular  leakage  from  the  wires  leading  to  the 
Holtz  machine.  This  trouble  was  made  more  difficult  to  control  by  un- 
favorable atmospheric  conditions,  the  work  being  carried  on  in  the  spring ; 
but  the  disturbances  due  to  leakage  were  largely  removed  by  using  large 
rods  in  place  of  wires,  and  by  avoiding  sharp  corners.  It  proved  advan- 
tageous, also,  to  run  the  Holtz  machine  at  full  speed  and  to  cut  down  the 
potential  difference  between  the  terminals  of  the  tube  by  means  of  resistance 
in  series.  The  necessary  resistance  was  furnished  by  a  tube  containing 
absolute  alcohol.  Flickering  persisted  to  some  extent  even  with  these 
precautions,  and  it  was  frequently  necessary  to  discard  the  results  of  several 
hours  work.  The  results  given  here  were,  however,  taken  under  favorable 
conditions.1 

It  did  not  prove  practicable  to  maintain  the  pressure  in  the  tube  constant 
during  the  time  necessary  for  spectrophotometric  measurements  through- 
out the  spectrum.  Owing  to  a  small  leak,  or  to  the  development  of  gas 
by  the  discharge,  the  pressure  gradually  increased  during  the  course  of  a 
series  of  readings,  so  that  at  the  end  of  about  an  hour  it  was  necessary  to 
reexhaust.  In  order  to  determine  the  luminescence  spectrum  corresponding 
to  constant  pressure  conditions  the  following  procedure  was  adopted: 

While  one  observer  recorded  the  readings  of  the  voltmeter  and  galva- 
nometer and,  when  necessary,  operated  the  pump,  the  other  made  settings 
of  the  spectrophotometer  as  rapidly  as  possible,  running  back  and  forth 
through  the  spectrum  until  the  whole  region  had  been  covered  a  number 
of  times.  Curves  were  then  drawn  for  each  wave-length  at  which  settings 
had  been  made  by  plotting  intensities,  as  measured  by  the  spectrophoto- 
meter, against  voltmeter  readings.  A  series  of  such  curves  is  shown  at  the 
left  in  Fig.  145,  which  refers  to  a  cadmium  sulphate  preparation.  These 
curves  all  have  the  same  general  shape,  and  in  most  cases  each  curve 
contains  enough  points  to  determine  its  form  with  considerable  definiteness. 
After  the  curves  had  been  drawn  as  accurately  as  possible  the  intensity 
corresponding  to  any  particular  voltmeter  reading  could  be  determined 
by  graphical  interpolation.  The  points  which  form  the  two  curves  to  the 
right  in  Fig.  145  (marked  I.  V.  M.  30  and  II.  V.  M.  17.5)  were  deter- 
mined in  this  way  for  the  voltmeter  readings  30  and  17.5  respectively. 
For  these  curves  the  abscissas  are  wave-lengths  expressed  in  fractions  of  ju 
as  indicated. 

The  procedure  in  the  case  of  our  experiments  with  willemite  (Fig.  146) 
and  Sidot  blende  (Fig.  147)  was  exactly  the  same  as  in  the  case  of  cadmium 
sulphate.  A  comparison  of  these  three  figures  shows,  however,  that  while 
the  voltage  intensity  curves  in  Figs.  145  and  146  are  similar,  being  in  each 
case  concave  toward  the  horizontal  axis,  the  corresponding  curves  in  Fig. 
148  are  almost  exactly  straight.  The  difference  results  from  the  fact  that  in 
the  experiments  corresponding  to  Figs.  145  and  146  the  current  through  the 
tube  diminished  rapidly  as  the  voltage  increased,  whereas  in  the  experi- 

'It  is  probable  that  the  trouble  was  due  to  electrostatic  charges  on  the  walls  of  the  tube.  A  larger  tube 
so  constructed  as  to  have  the  kathode  at  a  considerable  distance  from  the  walls  would  probably  have  been 
more  satisfactory. 


SPECTROPHOTOMETRIC    STUDY    OF   KATHODO-LUMINESCENCE. 


139 


merits  on  Sidot  blende  (Fig.  148)  the  conditions  as  regards  leakage  were 
more  favorable  and  the  current  remained  nearly  constant.  During  the 
observations  corresponding  to  Fig.  148  the  deflection  of  the  galvanometer 
varied  from  32  for  the  highest  voltmeter  reading  (33.8)  to  37  for  the  lowest 
voltmeter  reading  (11.7).  In  the  experiments  with  willemite  (Fig.  146) 
the  galvanometer  deflection  ranged  from  7  to  25 ;  and  in  the  experiments 
with  cadmium  sulphate  the  range  was  from  15  to  31. 

The  results  obtained  are  plotted  in  Figs.  145  to  148  and  are  further 
discussed  below. 

CdSO±-\-xM nSOt. — This  substance  was  prepared  by  C.  W.  Waggoner,  who 
has  determined  its  decay  curve  when  excited  by  the  iron  spark.1  Excited  by 
kathode  rays  it  gave  an  intense  yellow  fluorescence,  with  scarcely  observable 
phosphorescence.  Close  inspection  of  the  powder  showed  occasional  grains 


15        20       ZS       30 


56  60 


Fig.  145- 


CdSO«+*MnSO4.  The  curves  to  the  left  show  the  relation  between  discharge  potential  and  intensity  for 
different  wave-lengths.  Curves  /  and  //  show  the  luminescence  spectra  for  discharge  potentials  of 
1 1 ,600  volts  and  6,700  volts  respectively.  Curves  ///  and  IV  show,  to  a  larger  scale,  the  luminescence 
spectra  excited  by  ultra-violet  light  and  Roentgen  rays  respectively. 

which  glowed  with  an  orange  or  red  light.  At  the  end  of  our  experiments, 
after  the  powder  had  been  bombarded  by  the  kathode  rays  for  about  10 
hours,  the  surface  was  found  to  have  acquired  a  ruddy  brown  discoloration. 
Upon  standing  for  several  months  the  powder  scarcely  responded  at  all  to 
excitation  by  the  iron  spark,  but  recovered  its  activity  after  heating. 

Inspection  of  curves  /  and  //,  Fig.  145,  shows  that  the  form  of  the  fluo- 
rescence spectrum  is  independent  of  the  discharge  potential,  and  therefore 
of  the  velocity  of  the  kathode  rays,  since  it  is  clear  that  one  of  these  curves 
might  be  obtained  from  the  other  merely  by  changing  the  scale. 

JWaggoner,  Physical  Review,  xxvil,  p.  209,  1908.     See  also  Chapter  VII  of  this  memoir. 


14°  STUDIES   IN   LUMINESCENCE. 

The  fluorescence  spectrum  was  also  determined  for  excitation  by  the 
ultra-violet  rays  in  the  spectrum  of  the  iron  spark,  and  for  excitation  by 
Roentgen  rays,  the  results  being  shown  in  curves  III  and  IV,  respectively. 
The  luminescence  excited  in  both  of  these  rays  was  weak  and  the  spectrum 
could  be  explored  only  with  great  difficulty.  Curves ///  and  IV  are  drawn 
to  a  much  larger  scale  than  curves  /and  II  in  order  that  they  may  be  shown 
in  the  same  figure.  In  reality  the  intensity  at  the  maximum  of  curve  7 
is  200  times  as  great  as  the  corresponding  intensity  for  curve  III,  and 
500  times  as  great  as  the  maximum  for  curve  IV.  It  will  be  observed  that 
the  maximum  of  curve  IV  occurs  at  the  same  wave-length  as  in  the  case 
of  kathodo-luminescence.  In  the  case  of  photo-luminescence,  curve  ///, 
there  is  an  apparent  shift  toward  the  shorter  waves.  We  are  of  the  opinion, 
however,  that  this  shift  is  not  real,  but  due  to  errors  resulting  from  the 
extreme  faintness  of  the  spectrum. 

The  substance  continued  to  glow  for  a  long  time  when  excited  by  ultra- 
violet light,  but  after  excitation  by  kathode  rays  the  phosphorescence 
died  out  with  great  rapidity. 

WILLEMITE. 

The  specimen  which  was  tested  was  a  part  of  the  same  piece  that  had 
previously  been  used  in  determining  the  decay  of  phosphorescence  in  this 
substance.1  No  change  was  observable  in  the  specimen,  even  after  prolonged 
excitation. 

While  the  kathodo-luminescence  of  willemite  was  not  much  more  intense 
than  that  of  CdSO4,  the  photo-luminescence,  excited  as  before  by  the 
ultra-violet  rays  from  an  iron  spark,  was  found  to  be  much  brighter  than 
that  of  CdSC>4.  The  luminescence  excited  by  Roentgen  rays  was,  however, 
only  slightly  more  intense.  Curves  IV  and  V,  Fig.  146,  which  show  the 
spectral  distribution  for  the  photo-luminescence  and  the  Roentgen  lumi- 
nescence respectively,  are  plotted  to  a  larger  scale  than  the  curves  for 
kathodo-luminescence  on  the  same  figure,  as  it  would  otherwise  be  impossi- 
ble to  see  the  details  of  these  curves.  The  maximum  intensity  for  curve  / 
was  in  reality  120  times  as  great  as  that  for  curve  IV  and  1,500  times  as 
great  as  for  curve  V ' . 

Inspection  of  the  curves  of  Fig.  146  seems  to  justify  the  conclusion  that 
the  luminescence  spectrum  of  willemite  has  the  same  form  whether  the 
exciting  agent  is  ultra-violet  light,  Roentgen  rays,  or  kathode  rays.  The 
form  of  the  spectrum  is  also  seen  to  be  independent  of  the  potential  difference 
between  the  terminals  of  the  tube,  and  is  therefore  independent  of  the 
velocity  of  the  kathode  rays. 

When  excited  by  ultra-violet  light  the  willemite  used  in  these  experiments 
showed  bright  phosphorescence,  whose  rate  of  decay  was  so  slow  as  to  be 
comparable  with  that  of  Sidot  blende.  But  when  excited  by  kathode  rays 
the  phosphorescence  was  faint  and  disappeared  within  a  few  seconds.  The 
intense  brilliancy  of  the  kathodo-luminescence  during  excitation  makes  this 
difference  especially  striking. 


1  Nichols  and  Merritt,  Physical  Review,  xxin,  p.  37,  1906.     See  also  Chapter  IV  of  this  memoir. 


SPECTROPHOTOMETRIC   STUDY   OF    KATHODOLUMINESCENCE. 


141 


SIDOT  BLENDE. 

The  specimens  tested  were  pieces  of  the  same  screen  that  had  been  used 
in  the  work  on  Sidot  blende  described  in  Chapters  IV  and  V.1 

Our  first  measurements  of  the  kathode-luminescence  of  this  substance 
were  made  in  connection  with  this  earlier  work,  and  although  the  conditions 
were  not  so  definite  as  in  the  case  of  the  more  recent  investigation,  the 
result  of  one  exploration  of  the  spectrum,  shown  in  Fig.  147,  will  be  of 
interest  for  purposes  of  comparison.  Kathode  rays  were  developed  in 
these  experiments  by  connecting  the  tube  used  with  the  secondary  of  an 


160 


30  VM 


Fig.  146. — Willemite. 

The  curvesto  the  left  are  voltage-intensity  curves  for  different  wave-lengths.  Curves  /,  //,  and  ///  show  the 
luminescence  spectra  for  discharge  potentials  of  13,500  volts,  9,600  volts,  and  6,700  volts  respectively. 
Curves  /  V  and  V  show,  to  a  large  scale,  the  luminescence  spectra  excited  by  ultra-violet  light  and 
Roentgen  rays  respectively. 

induction  coil,  the  primary  of  which  was  supplied  with  alternating  current. 
The  discharge  was  made  approximately  unidirectional  by  placing  a  rectify- 
ing tube  in  series.  The  current  was  controlled  by  an  alcohol  resistance. 
The  current  used  being  smaller  than  that  employed  in  our  more  recent 
work,  less  difficulty  was  met  with  in  maintaining  constant  pressure  in  the 
tube.  That  the  pressure  was  nearly  constant  during  the  measurements 
corresponding  to  Fig.  147  is  shown  by  the  fact  that  it  was  possible  to  run 

'Nichols  and  Merritt,  Physical  Review,  xxi,  p.  247;  xxn,  p.  279;  xxm,  p.  37;  xxv,  p.  362. 


142 


STUDIES   IN   LUMINESCENCE. 


48 
I 

40 


SI 

I 


through  the  spectrum  a  second  time  with  practically  identical  results. 
The  points  marked  by  circles  in  Fig.  147  were  determined  first,  while 
those  marked  with  crosses  were  taken  afterwards,  with  no  change  in  the 
conditions,  as  a  check.  It  is  clear  that  the  spectrum  contains  two  over- 
lapping bands,  and  the  estimated  curves  for  the  separate  bands  are  indicated 
by  the  dotted  lines. 

Our  more  recent  measurements  with  Sidot  blende  were  made  by  the 
same  method  as  that  used  with  CdSO4  and  willemite.  The  results  are 
shown  in  Fig.  148. 

When  subjected  to  the  relatively  intense  kathode  rays  used  in  these  ex- 
periments the  Sidot  blende  was  found  to  undergo  a  rapid  change,  which  was 
manifested  both  by  a  discoloration  of  the  surface  and  by  a  diminution  in  the 

intensity  of  the  luminescence. 
In  Fig.  148,  curve  III  corre- 
sponds to  the  same  voltmeter 
reading  as  curve  7,  but  was 
taken  after  the  substance  had 
been  excited  continuously  for 
2  or  3  hours.  Not  only  is  the 
intensity  much  diminished  by 
prolonged  excitation,  but  the 
whole  character  of  the  lumines- 
cence spectrum  is  altered. 

The  band  at  about  0.5  /x, 
which  appears  in  Fig.  147  and 
in  curves  /  and  II  of  Fig.  148, 
appears  to  be  the  same  as  that 
excited  by  light  and  by  Roent- 
gen rays.1  The  band  at  about 
0.455  M.  is  also  excited  by 
Roentgen  rays.2  But  this  band 
is  either  not  present  in  the 
spectrum  excited  by  ultra- 
violet light,  or  is  masked  by 
the  other  bands  present  in 
that  spectrum.  The  lumines- 
cence of  Sidot  blende,  as  we  have  previously  pointed  out,  is  extremely  com- 
plex, and  the  green  band  at  about  0.51/1  is  the  only  one  which  can  be 
readily  isolated. 

When  excited  by  kathode  rays  the  phosphorescence  of  Sidot  blende  is 
faint  and  of  relatively  short  duration. 

The  infra-red  rays  obtained  by  interposing  a  thin  sheet  of  hard  rubber 
in  the  path  of  the  rays  from  an  arc  diminished  the  kathodo-luminescence 
of  Sidot  blende  only  slightly.  A  long  series  of  settings,  alternately  with 
and  without  infra-red  rays,  was  necessary  to  make  certain  that  any  effect 
was  produced  at  all.  Infra-red  rays  of  the  same  intensity  would  have  cut 
down  the  fluorescence  excited  by  light  to  one-half  its  normal  intensity.3 

'Nichols  and  Merritt,  Physical  Review,  xxi,  p.  247, 1905,  Figs.  30, 3 1 ,  and  35.    See  also  Chapter  III,  Fig.  40. 

2See  Fig.  41,  p.  44. 

'Physical  Review,  xxv,  p.  362,  1907.     See  also  Chapter  V  of  this  memoir. 


16 


.44    .46    .48      .50    .5?    ,54    .5fc   .SB/u  .60 

Fig.  147. — Sidot  blende. 


SPECTROPHOTOMETRIC    STUDY   OF    KATHODO-LUMINESCENCE. 


143 


DEPENDENCE  OF  KATHODO-LUMINESCENCE  UPON  CURRENT  AND 
DISCHARGE  POTENTIAL. 

Since  kathodo-luminescence  results  from  the  bombardment  of  the  lumi- 
nescent substance  by  the  kathode  rays  it  is  natural  to  expect  a  simple  relation 
between  the  intensity  of  luminescence  and  the  velocity  and  number  of  the 
kathode-ray  particles.  The  experiments  here  described  show  that  the  form 
of  the  luminescence  spectrum  is  independent  of  both  these  factors.  The 
experiments  show  also  that  the  intensity  of  luminescence  is  increased  by 
increasing  either  the  velocity  of  the  rays  or  the  current  in  the  tube.  But 
the  conditions  of  the  experiments  were  unfortunately  not  such  as  to  permit 
definite  conclusions  to  be  drawn  regarding  the  quantitative  relations. 


140 


120 


20       25       30V.M. 


.46 


.50 


.54 


.58/1 


Fig.  148.— Sidot  blende. 


The  curves  to  the  left  are  voltage-intensity  curves  for  different  wave-lengths.  Curves  /  and  //  show  the 
luminescence  spectra  for  discharge  potentials  of  n.6oo  volts  and  7,700  volts  respectively.  Curve/// 
shows  the  luminescence  spectrum  for  1 1,600  volts  after  the  substance  had  been  altered  by  the  action  of 
the  kathode  rays. 

Several  attempts  were  made  to  determine  the  relation  between  intensity 
and  current  at  a  constant  discharge  potential.  The  current  could  be  varied 
through  a  wide  range  either  by  changing  the  speed  of  the  Holtz  machine 
or  by  altering  a  resistance  in  multiple  with  the  tube.  The  results  make  it 
seem  probable  that  the  intensity  of  luminescence  is  proportional  to  the 
current ;  but  erratic  changes  in  the  conditions  in  the  tube,  due  probably  to 
static  charges  on  the  walls,  made  it  impossible  to  get  entirely  consistent 


144  STUDIES   IN   LUMINESCENCE. 

results.  For  a  time  the  conditions  would  apparently  remain  constant  and 
the  intensity  was  found  to  vary  in  the  same  ratio  as  the  current.  But 
some  source  of  disturbance  soon  developed,  so  that  the  next  observations 
were  discordant.  Some  change  in  the  form  of  tube  or  the  source  of  current 
supply  will  be  necessary  before  this  question  can  be  definitely  settled. 

Working  with  the  relatively  slow  kathode  rays  developed  by  ultra-violet 
light,  Lenard  has  found  the  intensity  of  luminescence,  7,  to  be  related  to  the 
discharge  potential,  V,  by  the  equation 


where  Q  is  the  density  of  the  kathode  stream,  C  a  constant,  and  Vo  a  mini- 
mum potential  below  which  no  luminescence  is  produced.1  The  existence 
of  a  sharply  defined  lower  limit  to  the  potential  that  is  capable  of  producing 
luminescence  is  called  in  question  by  Wehnelt,2  who  used  in  his  experiments 
a  much  greater  density  of  the  kathode  rays.  On  the  other  hand  a  definite 
lower  limit  to  the  velocity  has  been  found  by  Rutherford3  in  the  analogous 
case  of  luminescence  produced  by  the  a  rays  of  radium. 

Our  own  experiments  with  Sidot  blende  appear  at  first  glance  to  confirm 
the  conclusions  of  Lenard,  for  the  voltage-intensity  curves,  Fig.  148,  are 
straight  lines  cutting  the  horizontal  axis  at  a  point  corresponding  to  a  volt- 
meter reading  of  about  1  2  divisions.  In  the  case  of  the  experiments  with 
Sidot  blende  the  current  in  the  tube  varied  only  slightly.  We  are  not 
justified  in  concluding,  however,  that  the  density  of  the  kathode  stream 
was  constant,  for  the  fraction  of  the  current  that  is  carried  by  the  kathode 
rays  is  known  to  vary  as  the  vacuum  changes,  being  practically  zero  at 
high  pressures  and  nearly  100  per  cent  when  the  pressure  is  low.  In  our 
experiments  it  was  not  practicable  to  determine  the  amount  of  this  change. 
It  seems  scarcely  probable,  however,  that  the  change  was  unimportant. 
If  the  curves  of  Fig.  148  could  be  corrected  so  as  to  refer  to  a  constant 
density  of  the  kathode  stream  it  appears  probable  that  they  would  differ 
greatly  from  the  curves  plotted. 

CONCLUSIONS. 

In  so  far  as  it  is  possible  to  generalize  on  the  basis  of  the  small  number  of 
experiments  here  described,  the  following  conclusions  appear  to  be  justified  : 

i  .  In  cases  of  kathodo-luminescence  the  distribution  of  intensity  through- 
out each  band  of  the  luminescence  spectrum  is  independent  of  the  discharge 
potential,  and  therefore  independent  of  the  velocity  of  the  kathode  rays. 

2.  In  cases  where  a  band  is  capable  of  being  excited  by  light  and  by 
Roentgen  rays  as  well  as  by  kathode  rays,  the  form  of  the  band  and  the 
position  of  its  maximum  are  the  same  for  all  these  modes  of  excitation. 

We  have  shown  in  Chapters  III  and  IV  that  the  distribution  of  intensity 
in  any  given  band  is  independent  of  the  wave-length  of  the  exciting  light.4 
It  appears  probable,  therefore,  that  the  above  conclusion  may  be  stated 
more  broadly,  and  that  the  distribution  of  intensity  in  each  band  of  a 
luminescence  spectrum  is  independent  of  the  nature  of  the  exciting  agent 
by  which  the  luminescence  is  produced. 

'Lenard,  Annalen  der  Physik,  12,  p.  449,  i9°3- 

2Wehnelt,  Berliner  physikalische  Gesellschaft,  5,  p.  225,  1903. 

3Rutherford,  Radioactivity,  second  edition,  p.  547. 

'Nichols  and  Merritt,  Physical  Review,  XVHI,  p.  403;  xix,  p.  18. 


SPECTROPHOTOMETRIC    STU»Y   OF   KATHODO-LUMINESCENCE.  145 

3.  Phosphorescence  following  excitation  by  kathode  rays  is  less  intense 
and  more  fleeting  than  the  phosphorescence  excited  in  the  same  substance 
by  light. 

The  explanation  of  this  fact  is  probably  to  be  found  in  the  relatively 
slight  penetrating  power  of  the  kathode  rays.  The  excitation  is  thus  con- 
fined to  a  thin  layer  at  the  surface  of  the  active  substance,  while  in  the  case 
of  photo-luminescence  the  excitation  extends  to  a  considerable  depth.  As 
will  be  pointed  out  in  Chapter  XV  of  this  memoir,1  the  decay  of  phosphor- 
escence would  be  more  gradual  in  the  latter  case. 

4.  The  effect  of  infra-red  rays  upon  kathodo-luminescence  of  Sidot 
blende  during  excitation  is  small  compared  to  the  same  effect  upon  the 
photo-luminescence  of  this  substance  and  produces  a  barely  observable 
decrease  in  the  intensity  of  luminescence. 

'Nichols  and  Merritt.  Physical  Review,  xxvu.  p.  373,  1908. 


CHAPTER  X. 

ON  THE  ELECTRICAL  PROPERTIES  OF  FLUORESCENT  SOLUTIONS 

AND  VAPORS. 

THE  INFLUENCE  OF  FLUORESCENCE  UPON  ELECTRICAL  CONDUCTIVITY. 

The  effect  of  light  and  of  Roentgen  rays  upon  the  conductivity  of  solutions 
has  been  tested  by  Cunningham,1  in  the  case  of  five  solutions,  one  of  which, 
namely,  uranyl  nitrate,  was  fluorescent.  In  the  experiments  with  light  an 
arc  containing  iron  was  used  as  a  source,  and  the  solutions  were  contained 
in  a  cell  having  a  quartz  window,  the  object  being  to  obtain  intense  ultra- 
violet rays.  Although  some  indication  of  increased  conductivity  was  ob- 
served, the  observations  were  rendered  so  uncertain  by  the  heating  effect 
of  the  rays  from  the  arc  that  the  author  did  not  regard  the  results  as 
reliable. 

Experiments  of  a  similar  character  were  undertaken  by  Regner2  with 
especial  precautions  against  the  disturbances  due  to  the  heating  effect  of 
the  source.  No  change  in  conductivity  as  great  as  o.i  per  cent  could 
be  detected,  although  the  substances  tested  included  those  for  which 
Cunningham  had  found  indications  of  a  positive  result.  Several  fluorescent 
substances  were  tested  by  Regner,  also  with  negative  results. 

In  a  paper  by  Burke3  on  the  change  in  absorption  due  to  fluorescence, 
mention  is  made  of  preliminary  experiments  upon  the  conductivity  of 
fluorescent  substances ;  but  the  author  states  that  difficulties  were  met  with 
which  led  him  to  abandon  these  experiments  and  to  take  up  the  study  of 
absorbing  power  instead. 

Our  own  experiments  upon  this  subject  were  undertaken  with  the  feeling 
that  the  increase  in  conductivity  due  to  fluorescence  was  probably  very 
small,  comparable  rather  with  the  conducting  power  produced  in  gases  by 
the  action  of  Roentgen  rays  than  with  ordinary  electrolytic  conductivity 
in  solutions.  In  attempting  to  detect  so  small  an  effect  it  seemed  advisable 
to  work  with  solutions  in  which  the  normal  conducting  power  was  as  small 
as  possible.  In  the  case  of  all  the  substances  thus  far  tested  we  have  there- 
fore used  absolute  alcohol  as  a  solvent.4 

The  accurate  measurement  of  the  resistance  of  alcoholic  solutions  offers 
considerable  difficulty ;  for  while  the  conducting  power  is  too  great  to  permit 
the  employment  of  an  electrometer  method,  such  as  would  be  used  with 
gases,  it  is  too  small  to  be  determined  with  accuracy  by  the  methods  gen- 
erally used  with  aqueous  solutions.  No  benefit  was  to  be  gained  in  the 
present  experiments  by  increasing  the  cross-section  of  the  liquid  tested, 
and  so  reducing  its  resistance ;  for  if  the  thickness  exceeded  a  rather  small 


'J.  A.  Cunningham,  "An   attempt   to   detect   the   ionization  of  solutions  by  the  action  of  light  and 
Roentgen  rays,"  Proc.  Cambridge  Philosophical  Society,  n,  pp.  43 1-433.     1902. 
=Physikalische  Zeitschrift,  4,  862,  1903. 
'Burke,  Philos.  Transactions,  igiA,  p.  87. 
•In  the  experiments  of  Cunningham  and  Regner  the  solvent  was  water. 

147 


148 


STUDIES   IX   LUMINESCENCE. 


value  the  absorption  of  the  liquid  made  it  impossible  to  excite  the  whole 
mass  to  fluorescence.  After  trying  a  number  of  different  methods  without 
satisfactory  results  we  abandoned  the  attempt  to  accurately  measure  the 
total  resistance,  and  directed  our  attention  to  the  measurement  of  the 
change  in  resistance  due  to  illumination.  The  most  satisfactory  method 
that  we  have  found  for  this  purpose  is  that  of  the  ordinary  Wheatstone 
bridge,  using  a  direct  current  and  a  galvanometer  of  high  resistance. 

The  form  of  the  cell  used  to  contain  fluorescent  solutions  is  of  consider- 
able importance  in  its  bearing  upon  the  sensitiveness  of  the  method.  Among 
the  galvanometers  available  for  use  with  the  bridge,  that  which  seemed 
best  suited  for  the  work  was  one  having  a  resistance  of  about  10,000  ohms. 
It  was  therefore  desirable  to  have  the  resistance  to  be  tested  as  near  to 
this  value  as  other  conditions  would  permit.  On  the  other  hand,  the  layer 
of  liquid  tested  should  be  so  thin  that  it  can  be  excited  to  strong  fluores- 
cence throughout  its  thickness.  Although  we  did  not  find  it  possible  to 
satisfy  both  of  these  conditions,  they  were  approxi- 
mately met  by  the  form  of  cell  shown  in  diagram 
in  Fig.  149. 

The  figure  shows  both  a  perspective  view  and  a 
horizontal  section  of  the  cell.  A  thick  piece  of  plate- 
glass,  G,  was  cut  of  such  a  size  as  nearly  to  fill  the  cell 
and  was  held  in  position,  tightly  pressed  against  the 
two  electrodes  a,  b  (of  platinum  foil),  by  the  corks  C,  C. 
Although  the  drawing  is  in  other  respects  approximately 
to  scale  and  nearly  actual  size,  the  thickness  of  the 
electrodes  a,  b  is  greatly  exaggerated.  The  thickness 
of  the  layer  of  liquid  between  G  and  the  walls  of  the 
tube  was  determined  by  the  thickness  of  the  electrodes 
and  was  about  o.i  mm.  In  the  section  shown  in  Fig. 
149  the  portions  occupied  by  the  solution  are  left  un- 
shaded. The  length  of  the  electrodes  was  about  20 
mm.  and  their  distance  apart  2  mm. 

The  fluorescent  substances  tested  were  eosin,  fluor- 
escein,  rhodamin,  napthalin  roth,  and  cyanin.  As 
already  stated,  the  solvent  was  in  all  cases  absolute  alcohol.  The  solutions 
were  made  quite  concentrated,  so  that  fluorescence  was  confined  to  a  thin 
layer  at  the  surface.  The  concentration  was  so  adjusted  as  to  make  this 
fluorescent  layer  approximately  o.  i  mm.  thick. 

The  cell  containing  the  solution  to  be  tested  was  made  one  arm  (C)  of  a 
bridge.  The  resistance  of  the  arm  M  in  series  with  C  was  in  all  cases  9,000 
ohms,  while  the  resistance  of  the  third  arm,  R,  was  5,000  ohms.  The 
fourth  arm,  N,  was  varied  until  an  approximate  balance  was  obtained1 
and  the  apparent  resistance  was  computed  by  the  formula  C  =  MR/N. 
Current  was  furnished  by  two  gravity  cells  in  series. 

Polarization  in  the  fluorescent  solution  made  it  impossible  to  obtain  the 
true  resistance  by  this  method,  and  the  apparent  resistance  of  the  cell 
(computed  as  if  polarization  were  absent)  is  doubtless  in  error  by  50  per 

'These  resistances  were  chosen  not  because  they  gave  the  highest  attainable  sensitiveness,  but  because 
they  were  the  most  suitable  ones  that  were  available  at  the  time. 


Fig.  149. 


ELECTRICAL,  PROPERTIES  OF   FLUORESCENT  SOLUTION'S.  149 

cent  or  more.  But  the  sensitiveness  of  the  arrangement  to  changes  in  the 
resistance  of  the  test  cell  was  high.  In  order  to  avoid  disturbances  due 
to  variations  in  the  polarization  E.M.F.  it  was  found  necessary  to  keep  both 
the  battery  circuit  and  the  galvanometer  circuit  closed.  Even  the  small 
change  made  in  N  during  the  adjustment  of  the  balance  produced  a  con- 
siderable alteration  in  the  polarization,  so  that  the  adjustment  often  had  to 
be  continued  for  an  hour  or  more  before  the  needle  of  the  galvanometer 
was  sufficiently  steady  for  observations  to  be  begun.  During  this  time 
the  cell  was  protected  from  the  action  of  light  by  an  opaque  screen.  When 
the  conditions  had  become  steady,  or  more  frequently  when  the  motion 
of  the  galvanometer  needle  was  reduced  to  a  slow  uniform  drift,  the  screen 
was  removed  and  light  from  an  arc  was  allowed  to  fall  on  the  cell.  After 
the  effect  of  illumination  had  been  noted,  and  when  the  needle  had  again 
settled  down  to  a  steady  drift,  the  screen  was  replaced,  and  the  throw  of  the 
needle  in  the  opposite  direction  was  observed. 

In  the  measurement  of  so  small  an  effect  as  that  here  considered  it  is 
clear  that  the  heating  effect  of  the  rays  from  an  arc  is  likely  to  produce 
serious  errors,  for  the  change  in  conductivity  due  to  rise  in  temperature  is 
in  the  same  direction  as  the  change  that  we  are  attempting  to  detect.  For 
this  reason  the  use  of  the  direct  rays  of  the  arc,  even  at  the  distance  of  a 
meter,  was  entirely  out  of  the  question.  Even  when  a  water  cell  was  inter- 
posed in  the  path  of  the  rays  the  needle  was  displaced  through  several 
hundred  divisions.  That  this  movement  of  the  needle  was  due  to  rise  in 
temperature,  and  not  to  the  effect  sought,  was  indicated  by  the  fact  that  the 
change  persisted  after  the  rays  were  cut  off;  it  was  necessary  to  wait  at 
least  15  minutes  before  the  original  balance  was  approximately  restored. 

In  order  to  avoid  the  disturbances  due  to  rise  in  temperature  we  dispersed 
the  rays  from  the  arc  by  a  prism  and  used  only  those  portions  of  the  spec- 
trum that  were  most  effective  in  producing  fluorescence.  With  this  arrange- 
ment the  effects  observed  were  much  smaller  than  before,  but  they  were 
entirely  free  from  any  indication  of  temperature  changes.  Upon  removing 
the  screen,  so  as  to  illuminate  that  part  of  the  fluorescent  solution  lying 
between  the  electrodes,  a  throw  of  the  galvanometer  needle  was  observed, 
and  if  originally  free  from  drift  the  needle  vibrated  about  a  new  position  and 
finally  came  to  rest.  Continued  illumination  of  the  solution  produced  no 
increase  in  the  deflection.  Upon  replacing  the  screen  a  throw  in  the  oppo- 
site direction  occurred,  and  the  needle  finally  returned  to  its  original  reading. 
If  observations  were  made  while  the  needle  was  in  motion,  these  throws 
were  simply  superposed  upon  the  steady  drift.  In  this  case  the  effect  of 
illumination  was  measured  by  the  average  of  the  two  throws. 

So  far  as  we  were  able  to  judge,  the  effect  produced  by  light  reached  its 
full  value  at  once  and  ceased  as  soon  as  the  light  was  cut  off.  The  dis- 
turbance of  the  balance  of  the  bridge  was  always  such  as  to  indicate  an 
increase  in  the  conductivity  of  the  fluorescent  solution.  Upon  repeating 
the  experiments  with  rays  from  different  portions  of  the  spectrum  it  was 
found  that  a  change  in  conductivity  was  produced  only  by  those  rays  which 
were  able  to  excite  fluorescence  in  the  solution  tested.  And  so  far  as  we 
could  estimate  the  intensity  of  fluorescence  by  the  eye,  the  rays  which  gave 
the  most  intense  fluorescence  also  produced  the  greatest  change  in  con- 


150 


STUDIES  IN   LUMINESCENCE. 


ductivity.  In  the  case  of  eosin  we  were  able  to  follow  both  effects  to  the 
extreme  edge  of  the  violet,  while  illumination  by  the  red  of  the  spectrum 
produced  no  effect. 

In  order  to  form  an  estimate  of  the  magnitude  of  the  change  produced 
we  observed  in  each  case  the  throw  caused  by  increasing  or  decreasing  the 
resistance  N  by  one  ohm.  By  comparing  this  with  the  throw  due  to 
illumination,  it  was  possible  to  express  the  observed  change  as  a  fraction 
of  the  normal  (apparent)  resistance.  The  results  are  given  in  Table  19. 
The  light  used  to  illuminate  the  solution,  in  the  case  of  the  measurements 
included  in  the  table,  was  that  which  produced  the  brightest  fluorescence. 

We  have  also  tested  in  the  same  way  one  solution  that  was  not  fluorescent, 
namely,  an  alcoholic  solution  of  fuchsin.  The  result  was  entirely  negative. 
No  change  in  resistance  due  to  illumination  could  be  found  in  any  part  of 
the  spectrum,  although  the  sensitiveness  of  the  bridge  was  such  that  a 
change  of  0.008  per  cent  could  have  been  detected.  The  solution  was  an 
old  one  and  had  probably  been  made  with  commercial  alcohol,  since  the 
resistance,  90,000  ohms,  was  much  lower  than  that  of  the  other  solutions. 
For  this  reason  we  are  not  inclined  to  look  upon  this  single  experiment  with 
a  non-fluorescent  solution  as  possessing  much  significance. 

TABLE  19. 
Increase  in  electrical  conductivity  due  to  fluorescence. 


Increase  in 

Apparent 

Throw  pro- 

Throw pro- 

apparent 

Substance. 

N 

resistance 

duced  by 

duced  by 

conductivity 

of  solution. 

illumination. 

increasing  N 

due  lo 

by  I  ohm. 

illumination. 

mm. 

mm. 

per  ct. 

Kosin  

155 

3OO,OOO 

57 

}} 

I  .  I 

Nap  thalin  roth..  . 

45 

1,000,000 

1 

39 

O.O5 

Fluorescein  

300 

1  50,000 

IO 

ao 

O    I  I 

Rhodamin  

360 

I25,OOO 

2O 

4O 

O.  14 

Cyanin  

1  70 

270,000 

4O 

45 

O    52 

In  the  preceding  description  of  the  experiments  upon  fluorescent  solu- 
tions we  have  ascribed  the  observed  movements  of  the  galvanometer  needle 
to  a  change  in  the  resistance  of  the  solution.  The  galvanometer  deflections 
undoubtedly  indicate  a  disturbance  in  the  balance  of  the  bridge;  but  this 
disturbance  might  equally  well  result  from  a  diminution  in  the  E.M.F. 
of  polarization.  Without  special  modifications  the  method  does  not  permit 
a  separation  of  these  two  effects.  In  the  case  of  eosin,  however,  special 
experiments  were  made  bearing  upon  this  point. 

The  experiments  referred  to  were  of  two  kinds .  In  the  first  of  these  a  bridge 
method  was  employed  as  before.  The  solution  was  contained  in  a  glass  tube 
originally  about  5  mm.  in  diameter.  A  portion  of  this  tube  had  been  drawn 
down  to  a  diameter  of  about  i  mm.  and  the  current  passed  through  the 
liquid  contained  in  this  contracted  part.  The  platinum  electrodes  were 
placed  in  the  larger  part  of  the  tube  at  each  end.  We  were  thus  able  (i) 
to  illuminate  the  solution  about  either  electrode  while  the  remainder  of  the 
tube  remained  dark;  or  (2)  to  illuminate  the  contracted  part  of  the  tube 


ELECTRICAL   PROPERTIES   OF   FLUORESCENT   SOLUTIONS. 


while  screening  the  electrodes.  In  the  latter  case  the  effect  was  observed, 
as  in  the  experiments  already  described.  In  the  former  case  only  a  very 
small  change  could  be  detected. 

The  other  experiments  with  eosin  that  bear  upon  the  question  of  polari- 
zation were  of  an  entirely  different  character,  and  represent  one  of  our 
many  early  attempts  to  devise  a  satisfactory  method  of  attacking  the 
general  problem.  Fig,  150  shows  both  the  form  of  tube  used  to  contain 
the  solution  and  a  diagram  of  the  connections.  The  platinum- wire  elec- 
trodes a,  b  were  connected  to  the  terminals  of  the  secondary  of  an  induction 
coil,  /,  and  to  the  two  pairs  of  quadrants  of  an  electrometer,  E.  The  elec- 
trode C  at  the  center  of  the  tube  was  connected  to  the  needle.  The 
primary  of  the  induction  coil  was  excited  by  an  alternating  current  of  120 
cycles.  When  the  solution  was  not  illuminated  the  electrometer  needle 
stood  nearly  at  its  zero.  Under  these  circumstances  illumination  of  the 
upper  half  of  the  tube  pro- 
duced a  deflection  in  one 
direction,  while  the  illumina- 
tion of  the  lower  half  gave  an 
approximately  equal  deflec- 
tion in  the  opposite  direction. 
The  direction  of  the  deflection 
in  each  case  was  such  as  to 
indicate  a  decrease  of  resist- 
ance. The  spectrum  of  the 
arc  was  used  in  these  experi- 
ments as  in  those  made  by  the 
bridge  method;  and  the  evi- 
dence that  the  effect  was  due 
to  fluorescence,  and  not  to  rise 
of  temperature,  was  as  con- 
clusive as  in  the  experiments 
previously  described.  The 
method  was  less  sensitive, 
however,  and  was  therefore  used  only  in  the  case  of  eosin. 

Shortly  after  the  announcement  of  the  results  just  described,1  Camichel,2 
in  a  paper  in  the  Journal  de  Physique,  reported  a  series  of  measurements 
with  entirely  negative  results,  and  suggested  that  our  precautions  against 
heating  were  insufficient  and  that  the  apparent  change  of  conductivity 
might  be  thus  explained.  It  seemed  desirable,  therefore,  particularly  as 
our  experiments  had  been  merely  preliminary  and  prospective,  to  investi- 
gate further.  Dr.  Percy  Hodge,  at  our  suggestion,  took  the  matter  in  hand 
and  it  was  found,  as  will  appear  from  the  summary  of  his  work  given  in  the 
second  part  of  this  chapter  and  from  a  later  paper  by  Goldman,3  that  the 
effects  of  light  were  much  more  complicated  than  had  been  suspected. 


Fig.  150. 


'Nichols  and  Merritt,  Physical  Review,  xix,  p.  415,  1904. 
2Camichel,  Journal  de  Physique,  iv,  1905. 
3(5oldman,  Annalen  der  Physik,  vir,  p.  322,  1908. 


152 


STUDIES   IN   LUMINESCENCE. 


PHOTO-ACTIVE  CELLS  WITH  FLUORESCENT  ELECTROLYTES  (DR.  HODGE'S 

EXPERIMENTS).1 

Before  going  on  to  any  further  systematic  investigation,  Dr.  Hodge 
attempted  to  repeat  as  nearly  as  possible  some  of  the  experiments  de- 
scribed in  earlier  portions  of  this  chapter. 

The  substance  first  chosen  for  experiment  was  eosin,  and  nearly  all  the 
results  to  be  described  were  obtained  with  this  substance.  As  a  solvent 
absolute  alcohol  was  used,  and  in  most  cases  the  solution  was  saturated  at 
room  temperature. 

The  essentials  of  the  apparatus  used  in  these  preliminary  tests  are  shown 
in  Fig.  151.  The  source  of  light  A  was  a  Schuckert  arc  lamp  inclosed  in 
the  light-tight  cover  which  is  used  with  the  lamp  for  projection.  After 

passing  through  a  pair  of  condensers  the 
beam  of  light  illuminated  the  slit  S,  then 
went  to  a  water  cell  W,  and  thence  to  a  lens 
and  crown-glass  prism,  finally  forming  a 
spectrum  on  the  front  of  the  light-tight  box 
D,  containing  the  cell  C.  D  was  provided 
with  a  sliding  front  by  which  the  light  could 
be  admitted  to  the  cell  C. 

For  measuring  the  change  of  resistance 
an  ordinary  Wheatstone  bridge  was  used,  the 
cell  being  placed  in  one  arm.  The  source  of 
current  was  a  battery  of  gravity 
cells,  the  number  of  which  was 
varied,  as  will  be  seen  later.  A 
very  sensitive  Sullivan  d'  Ar- 
son val  galvanometer  of  i ,  100  ohms 
was  used  with  the  bridge.  The 
galvanometer  was  fitted  with  a 
concave  mirror  of  4  feet  focus, 

"    A 

and   its   constant   with  the  scale 
*ig-  152.    at   that   distance  was    9oXio~n 

ampere  per  millimeter  deflection. 

The  cell  used  to  contain  the  solution  was  a  small  rectangular  glass  cell 
6  cm.  deep  and  i  cm.  from  front  to  back  on  the  inside.  In  this  were  placed 
side  by  side,  and  at  a  distance  of  approximately  a  millimeter  apart,  two  strips 
of  platinum  foil  o.i  mm.  in  thickness,  5  cm.  long,  and  3  mm.  wide.  Behind 
these  in  the  cell  was  placed  a  piece  of  plate  glass,  holding  the  electrodes 
pressed  against  the  front  wall  of  the  cell,  and  itself  held  in  place  by  brass 
springs  between  it  and  the  rear  wall  of  the  cell.  A  section  of  the  cell  is 
shown  in  Fig.  152,  the  thickness  of  the  electrodes  and  other  dimensions  of 
the  cell  being  considerably  exaggerated.  The  dispersion  of  the  prism  gave 
a  visible  spectrum  at  the  point  where  the  cell  was  placed  about  4  cm.  wide. 
Thus  the  portion  of  the  spectrum  illuminating  the  narrow  space  i  mm.  wide 
between  the  electrodes  was  quite  small,  and  it  was  easy  to  select  the  part 
of  the  spectrum  producing  maximum  fluorescence  without  including  the 


'This  section  is  a  summary  of  a  paper  by  Dr.  Percy  Hodge;  see  Physical  Review,  xxvi,  p.  540;  and 
xxvm.  p.  25. 


ELECTRICAL  PROPERTIES 'OF  FLUORESCENT  SOLUTIONS.       153 

rays  of  longer  wave-length  which  would  be  likely  to  produce  heat  disturb- 
ances. In  the  case  of  the  eosin  cell  the  light  used  was  at  the  infra  edge  of 
the  green. 

When  the  cell  was  first  set  up  great  difficulty  was  experienced  in  obtaining 
a  balance,  the  drift  toward  higher  values  of  the  resistance  being  quite  rapid 
and  continuing  for  several  hours.  This  was  at  first  thought  to  be  due 
entirely  to  polarization,  but  it  was  found  that  if  the  top  of  the  cell  was 
covered  so  tightly  that  evaporation  was  prevented  the  drift  lasted  for  only 
a  short  time,  after  which  a  comparatively  steady  condition  was  reached, 
and  accurate  measurements  were  possible.  At  no  time  was  the  cell  abso- 
lutely free  from  a  very  slow  drift,  always  toward  higher  resistance,  and 
probably  due  partly  to  polarization  and  partly  to  a  very  gradual  decom- 
position of  the  electrolyte.  Evidence  of  the  latter  was  seen  in  a  very  narrow 
strip  of  colorless  liquid  which  was  observed  along  the  inner  edge  of  the 
anode  after  several  hours  of  application  of  the  current.  On  account  of 
this  decomposition  most  of  the  tests  were  made  with  cells  which  had  been 
freshly  set  up,  the  electrodes  being  removed  and  carefully  cleaned  between 
tests. 

The  method  of  the  experiment  was  as  follows.  After  the  apparatus  had 
been  set  up  and  the  cell  placed  in  its  dark  box  the  slide  was  raised  and  the 
cell  so  adjusted  in  the  spectrum  that  the  liquid  between  the  electrodes 
became  brilliantly  fluorescent.  The  slide  was  then  dropped,  shutting  off 
the  light,  and  a  balance  was  obtained  with  the  bridge.  The  resistance 
of  RI  (see  Fig.  151)  was  in  nearly  every  case  10,000  ohms,  that  of  Rs  was 
50,000  ohms,  and  J?4  was  adjusted  to  suit  the  resistance  to  be  measured. 
As  it  was  impossible  to  set  the  electrodes  at  exactly  the  same  distance  from 
each  other  when  the  cell  was  set  up  at  various  times  the  apparent  resistances 
differed  from  each  other  greatly  during  different  sets  of  readings.  Also  the 
polarization  E.M.F.  in  the  case  of  eosin  was  found  to  be  over  two  volts. 
Therefore  when  two  gravity  cells  were  used  the  apparent  resistance  was 
much  higher  than  when  four  were  used. 

After  the  cell  had  stood  in  darkness  long  enough  to  obtain  a  fairly  steady 
condition  of  the  bridge,  the  slide  was  suddenly  removed  and  the  cell 
illuminated. 

With  two  volts  as  the  applied  E.M.F.  the  result  was  an  immediate  and 
very  large  decrease  of  resistance,  the  amount  of  resistance  change  required 
in  the  variable  arm  to  restore  a  balance  indicating  as  high  as  10  or  15  per 
cent  increase  of  conductivity.  When  the  light  was  shut  off  the  cell  returned 
immediately  to  nearly  the  same  resistance  that  it  had  before  illumination. 
The  test  was  repeated  many  times  and  on  different  days,  and  always  with 
about  the  same  results.  As  the  effect  was  much  larger  than  any  that  had 
been  anticipated  it  seemed  advisable  to  see  if  it  could  in  any  way  be  due  to 
heating  of  the  liquid.  With  this  end  in  view  the  temperature  coefficient 
of  the  solution  was  determined  in  the  following  manner:  A  balance  was 
obtained  at  the  temperature  of  the  room.  Then  the  cell  was  surrounded 
with  ice  water  and  a  balance  again  obtained.  Lastly  the  ice  water  was 
drawn  off  and  the  cell  allowed  to  regain  the  temperature  of  the  room.  The 
first  and  last  values  of  the  apparent  resistance  were  in  turn  subtracted  from 
the  second  and  then  averaged.  The  result  showed  a  temperature  coefficient 


154  STUDIES   IN   LUMINESCENCE. 

between  zero  and  20  degrees  of  not  more  than  i  .5  per  cent  per  degree.  The 
effect  of  the  light  was  also  tried  while  the  cell  was  in  ice  water  and  found 
to  be  apparently  about  as  large  as  at  room  temperature.  These  tests 
seemed  to  preclude  the  possibility  of  any  considerable  portion  of  the  effect 
being  attributable  to  heat.  An  additional  reason  why  the  effect  should 
not  be  attributed  to  heat  was  mentioned  on  page  1 49  of  this  chapter,  namely, 
the  fact  that  the  change  of  conductivity  took  place  within  an  exceedingly 
short  time  after  the  light  was  thrown  on  the  cell,  and  after  the  light  was 
shut  off  the  cell  immediately  resumed  its  former  state.  This  fact  was  con- 
firmed in  the  course  of  these  experiments,  the  time  required  for  the  change 
to  take  place  being  apparently  considerably  less  than  a  second,  and  the 
decay  of  the  effect  taking  about  the  same  time. 

A  series  of  tests  was  then  made  with  the  same  cell  and  solution,  but  using 
four  gravity  cells  in  series  as  the  source  of  E.M.F.  A  difference  of  effect 
from  that  obtained  with  two  cells  was  expected,  as  with  two  cells  only  a 
very  small  current  could  possibly  have  passed  through  the  cell,  since  the 
applied  E.M.F.  was  not  equal  to  the  E.M.F.  of  polarization,  while  with 
four  volts  the  current  flowing  was  of  considerable  magnitude.  The  effect 
anticipated  was  one  similar  to  the  first  but  of  smaller  magnitude.  When 
the  light  was  thrown  on  the  cell,  however,  the  surprising  result  was  obtained 
of  a  large  increase  of  resistance  instead  of  a  decrease  as  in  the  former  experi- 
ments. The  effects  were  produced  as  promptly  as  those  of  the  preceding 
experiments  and  died  away  nearly  as  quickly. 

Here  was  something  quite  unexpected.  If  the  remarkable  decrease  of 
resistance  observed  in  the  first  experiments  was  due  to  electrons  set  free  by 
ionization  of  the  solution  accompanying  the  phenomenon  of  fluorescence 
it  seemed  highly  improbable  that  an  increase  of  resistance  would  be  pro- 
duced by  light  under  any  conditions  whatever.  Further,  both  effects  were 
entirely  too  large  to  be  satisfactorily  accounted  for  by  any  theory  anal- 
ogous to  that  applied  to  the  phenomena  of  ionization  in  gases ;  phenomena 
which  it  might  be  supposed  would  very  probably  accompany  fluorescence. 

Again,  if  the  decrease  of  resistance  in  the  first  case  were  due  to  ionization 
of  the  nature  of  that  produced  in  gases,  it  should  be  greater  in  dilute  solu- 
tions than  in  concentrated,  since,  owing  to  the  greater  penetration  of  the 
light  into  a  dilute  solution  the  number  of  molecules  affected  would  be  as 
great  as  in  a  concentrated  solution,  while,  owing  to  the  greater  distance 
between  the  molecules,  the  mean  free  path  of  the  ions  would  be  greater  and 
therefore  recombination  less  rapid.  A  number  of  experiments  were  made 
by  diluting  the  saturated  solution  with  two,  four,  and  six  parts  by  volume  of 
alcohol,  and  the  effects  were  found  to  be  greatly  diminished,  so  that  even  in 
the  solution  with  two  parts  of  alcohol  there  was  a  change  of  not  more  than 
i  per  cent  in  resistance. 

In  order  to  vary  the  conditions  it  was  thought  worth  while  to  try  next 
a  cell  in  which,  as  in  Regner's  experiments,  the  liquid  could  be  kept  moving 
past  the  electrodes  while  being  illuminated.  All  possible  heat  effects 
would  be  in  this  way  eliminated,  and  some  light  might  be  thrown  upon  the 
nature  of  the  phenomena  which  would  not  be  brought  out  when  the  liquid 
was  at  rest.  A  cell  was  therefore  prepared  in  the  following  manner,  again 
copy  the  designing  of  one  described  on  page  150. 


ELECTRICAL   PROPERTIES   Of   FLUORESCENT   SOLUTIONS.  155 

A  glass  tube  of  5  mm.  internal  diameter  was  drawn  down  at  its  middle 
point  to  a  capillary  about  a  millimeter  in  outside  diameter  and  1.5  cm.  in 
length.  Into  the  larger  parts  of  the  tube  at  each  end  were  led  platinum 
wires  in  the  form  of  spirals  to  serve  as  electrodes.  The  upper  end  of  the 
tube  was  attached  to  a  tubulated  bottle  to  serve  as  a  reservoir  for  the  liquid. 
To  the  lower  end  was  attached  a  second  capillary  tube  to  hold  the  liquid  back 
and  keep  the  first  capillary  full.  With  this  apparatus  the  liquid  could  be 
illuminated  while  flowing  through  the  capillary  at  any  rate  desired.  Numer- 
ous tests  were  made  with  the  apparatus,  and  in  all  cases  the  results  were 
entirely  negative.  The  form  of  the  cell  was  such,  however,  that  its  resist- 
ance was  much  larger  than  that  of  any  of  the  cells  previously  tried,  and 
as  the  sensitiveness  of  the  bridge  was  much  less  under  these  conditions,  the 
results  were  looked  upon  with  some  suspicion. 

Another  form  of  circulation  cell  was  therefore  devised  which  was  free 
from  the  above  objection. 

A  piece  of  glass  tubing  was  obtained  whose  internal  cross-section  was 
almost  that  of  a  figure  8,  except  that  where  the  two  loops  of  the  8  would 
cross  one  another  there  was  an  opening  between  them  about  0.3  mm.  in 
width.  In  a  short  length  of  this  tubing  were  placed  two  No.  16  platinum 
wires,  each  having  a  length  of  5  cm.  The  wires  were  parallel  to  one  another 
in  the  two  loops  of  the  8,  and  from  the  end  of  each  a  small  platinum  wire 
was  led  out  through  the  walls  of  the  tube.  There  was  thus  left  (see  Fig. 


./TVTV-.. 


Fig.  153-  Fig.  154.  Fig.  155. 

153,  which  shows  an  enlarged  cross-section  of  the  cell)  a  narrow  space 
between  the  electrodes  through  which  the  liquid  could  flow,  the  electrodes 
themselves  almost  filling  the  loops  of  the  8.  Below  the  electrodes  the  tube 
was  drawn  down  to  a  coarse  capillary  to  make  it  possible  to  keep  the  space 
between  the  electrodes  filled  with  liquid.  The  upper  end  of  the  tube  was 
attached  to  a  tubulated  bottle  as  in  the  last  experiment.  By  illuminating 
the  contracted  part  of  the  tube  between  the  electrodes  a  moving  strip  of 
liquid  about  0.3  mm.  wide  and  o.  i  mm.  from  front  to  back  could  be  excited 
to  fluorescence,  while  the  current  flow  was  from  side  to  side  of  the  strip  at 
right  angles  to  its  length. 

Upon  connecting  the  cell  to  the  bridge  its  resistance  reached  a  steady 
value  almost  immediately  after  the  current  was  turned  on,  and  retained 
the  same  value  as  long  as  the  motion  of  the  liquid  continued. 

The  movement  of  the  liquid  was  quite  rapid,  the  time  required  to  empty 
the  400  c.c.  reservoir  being  about  10  minutes.  As  the  whole  space  between 
the  electrodes  held  not  more  than  o.i  c.c.  a  complete  change  of  liquid 
between  the  electrodes  could  not  have  occupied  more  than  a  fifth  of  a 
second. 

Numerous  tests  were  made  with  this  apparatus,  but  in  no  case  was  there 
any  evidence  of  a  change  of  conductivity  while  the  liquid  was  in  motion. 
When,  however,  the  lower  end  of  the  tube  was  stopped  the  effects  reappeared, 
though  they  were  not  as  marked  as  in  the  original  type  of  cell.  The  sen- 


156  STUDIES  IN  LUMINESCENCE. 

sibility  of  the  bridge  as  used  in  these  tests  was  at  least  o.o  i  per  cent.  There- 
fore it  seems  fair  to  conclude  that  there  could  not  have  been  any  appreciable 
effect  due  to  electrons  set  free  at  the  instant  fluorescence  began  in  any  part 
of  the  liquid,  unless  this  effect  required  a  considerable  time  to  make  itself 
known. 

Attention  was  next  given  to  the  effects  at  the  two  electrodes  separately 
and  to  the  liquid  between  them,  to  find  out,  if  possible,  just  where  the  phe- 
nomena took  place  which  caused  the  changes  in  conductivity. 

As  nothing  but  negative  results  were  to  be  obtained  from  a  circulation 
cell  the  original  type  of  cell  was  again  adopted,  and  in  the  front  of  the 
box  D  of  Fig.  151  was  placed  a  screen  with  a  vertical  slit  cut  in  it  about  4 
mm.  wide.  By  placing  the  cell  behind  this  screen  in  such  a  manner  that 
only  one  electrode  could  be  seen  from  the  front  of  the  box  through  the  slit, 
it  was  possible  to  observe  the  effect  of  illuminating  one  electrode  at  a  time, 
the  other  being  in  darkness  behind  the  screen.  With  this  arrangement  and 
two  gravity  cells  the  effect  was  tried  on  anode  and  kathode  in  turn.  When 
the  anode  was  exposed  the  effect  was  very  small,  while  on  the  contrary 
when  the  kathode  was  exposed  the  effect  was  found  to  be  about  as  large  as 
when  both  were  exposed.  As  some  of  the  liquid  between  the  electrodes 
was  illuminated  in  either  case  it  was  not  certain  whether  the  effect  was  pro- 
duced only  at  the  electrode  or  whether  the  liquid  at  a  distance  from  the 
electrode  also  played  its  part  in  it.  Therefore  the  question  as  to  what 
would  happen  when  the  electrodes  were  both  covered  and  the  liquid  between 
them  exposed  had  next  to  be  settled. 

To  this  end  several  different  arrangements  of  the  cell  were  tried.  The 
first  tests  were  with  the  same  electrodes  which  had  been  used  in  the  earlier 
type  of  cell,  but  in  front  of  each  of  them  was  placed  a  thin  strip  of  hard 
rubber  0.15  mm.  thick,  projecting  just  beyond  the  platinum  strip  so  as  to 
completely  screen  it  from  the  light,  but  so  as  to  leave  about  80  per  cent  of 
the  liquid  between  the  electrodes  exposed  to  the  light. 

With  this  arrangement  no  effect  could  be  obtained  by  illumination.  How- 
ever, it  was  felt  that  the  test  was  not  conclusive,  for  the  total  thickness  of 
the  film  of  liquid  acting  as  conductor  between  the  electrodes  was  that  of 
both  the  electrode  and  the  rubber  strip,  and  since  the  absorption  coefficient 
of  a  saturated  solution  of  eosin  is  very  large  it  was  not  at  all  certain  that 
the  whole  thickness  of  the  layer  was  illuminated. 

To  overcome  this  difficulty  another  type  of  cell  was  constructed  after  the 
following  manner: 

A  block  of  hard  rubber  was  sawed  of  such  shape  and  size  as  nearly  to  fit 
the  rectangular  cell  used  in  most  of  the  experiments.  The  block  was  then 
clamped  to  a  block  of  ebonite  and  with  a  f -inch  drill  two  holes  were  bored 
between  the  blocks  in  such  a  manner  as  to  leave  in  each  block  a  pair  of 
grooves  of  semicircular  cross-section,  and  separated  from  each  other  by  a 
narrow  strip  approximately  2  mm.  wide.  The  block  that  had  been  sawed 
to  fit  the  cell  was  then  placed  in  the  cell  and  two  L-shaped  electrodes  were 
placed  in  the  grooves  in  the  positions  shown  in  Fig.  155,  which  illustrates 
a  section  of  the  cell  with  block  and  electrodes  in  place.  It  will  be  seen 
from  the  figure  that  the  electrodes  hold  the  ebonite  block  back  from  the 
front  wall  of  the  cell,  so  as  to  leave  a  film  of  liquid  of  the  thickness  of  the 


ELECTRICAL,  PROPERTIES   OF   FLUORESCENT  SOLUTIONS.  157 

platinum  strips  between  the  two  grooves  when  the  cell  is  filled  with  liquid. 
The  film  of  liquid  between  the  grooves  could  be  perfectly  illuminated,  was 
isolated  from  the  electrodes,  and  at  the  same  time  included  a  large  part  of 
the  total  resistance. 

Careful  tests  were  made  with  this  cell  and  the  results  were  entirely  nega- 
tive, though  the  resistance  of  the  cell  was  quite  low  compared  with  others 
that  had  been  tried,  averaging  about  40,000  ohms  with  saturated  solution, 
and  the  sensibility  of  the  bridge  was  as  high  as  0.005  Per  cent.  The  con- 
ditions within  a  few  minutes  after  it  was  set  up  were  very  steady  indeed, 
and  a  slight  change  of  conductivity  would  have  been  readily  detected. 

In  addition  to  the  saturated  solution  a  solution  of  five  parts  alcohol  to 
one  of  saturated  eosin  solution  was  tried  with  both  2  and  4  volts  E.  M.  F., 
but  likewise  with  entirely  negative  results.  From  the  results  of  these  last 
experiments  it  was  evident  that  a  change  of  conductivity  in  the  liquid  itself, 
except  at  or  very  near  the  electrodes,  did  not  take  place  when  fluorescence 
was  produced. 

It  had  frequently  been  observed  that,  however  smooth  the  strips  of 
platinum  were  when  they  were  placed  in  the  cell,  and  however  tightly  they 
were  clamped  against  the  front  wall,  when  the  solution  was  poured  in  a 
thin  film  of  liquid  immediately  crept  up  between  the  electrodes  and  the 
front  wall  of  the  cell.  There  seemed,  therefore,  to  be  two  regions  in  which 
the  effects  might  be  produced,  either  at  the  inner  edges  of  the  electrodes  or 
in  the  region  in  front  of  the  electrodes,  or  in  both  places  at  once. 

From  a  variety  of  minor  indications  which  had  been  noticed  from  time 
to  time  it  seemed  probable  that  the  main  effect  was  to  be  looked  for  in 
the  thin  film  and  not  at  the  edges  of  the  electrodes.  In  order  to  test  the 
matter  the  effect  was  first  tried  of  placing  thin  strips  of  ebonite  in  front  of 
the  electrodes  as  in  one  of  the  previous  experiments,  but  instead  of  cover- 
ing the  electrodes  the  ebonite  strips  were  only  allowed  to  cover  the  extreme 
outer  edges  of  the  platinum  strips,  thus  leaving  nearly  the  whole  of  the 
electrodes  exposed  to  the  light,  but  behind  a  layer  of  liquid  a  number  of 
times  as  thick  as  the  capillary  film  formed  when  the  electrodes  were  against 
the  front  wall  of  the  cell.  The  change  of  conductivity,  though  still  present, 
was  found  to  be  very  greatly  diminished. 

As  mentioned  in  connection  with  a  previous  experiment,  the  reduction 
of  the  effect  might  easily  have  been  due  to  the  absorption  of  the  light  by 
the  thick  layer  of  liquid  in  front  of  the  electrodes,  so  that  fluorescence  did 
not  occur  at  the  surface  of  the  platinum  strips. 

A  second  arrangement  was  then  tried  in  which  the  platinum  strips  were 
separated  from  the  front  wall  of  the  cell  by  strips  of  lantern-slide  cover- 
glass  a  millimeter  in  thickness.  The  inner  edge  of  each  electrode  (see  Fig. 
155)  was  allowed  to  project  slightly  beyond  the  edge  of  the  glass.  Thus 
the  capillary  film  between  the  electrode  and  strip  of  cover-glass  could  be 
illuminated  while  the  inner  edge  of  the  electrode  was  completely  screened 
from  the  light  by  a  thick  layer  of  liquid.  When  the  light  was  turned  on 
this  cell  the  effects  were  found  to  be  present  in  as  great  a  degree  as  when 
the  whole  of  the  electrode  was  illuminated. 

Lastly,  the  platinum  strips  were  again  placed  against  the  front  wall  of  the 
cell  and  the  inner  edges  covered,  while  the  rest  of  the  electrodes  was  illumi- 
nated. Again  the  effect  was  present,  as  was  to  be  expected. 


158  STUDIES   IX   LUMINESCENCE. 

The  conclusion  arrived  at  from  the  various  tests  was  that,  in  the  first 
place,  no  effect  is  produced  by  light  upon  the  conductivity  of  a  fluorescent 
solution  unless  a  region  very  near  the  electrodes  is  illuminated;  and  that 
further,  this  effect  at  the  electrodes  is  only  produced  in  a  very  thin  film  of 
the  liquid  immediately  in  contact  with  the  surface  of  the  plates. 

It  may  be  well  to  state  that,  while  no  reference  has  thus  far  been  made 
to  a  variation  of  the  effects  in  different  parts  of  the  spectrum,  frequent  tests 
were  made  in  all  of  the  experiments  described,  to  see  if  the  effects  could  be 
produced  by  parts  of  the  spectrum  which  did  not  produce  fluorescence. 
In  all  of  these  tests  the  effects  obtained  were  either  very  feeble  or  entirely 
lacking.  Thus,  in  the  red,  where  the  energy  of  the  arc  is  the  greatest,  but 
where  no  fluorescence  was  to  be  detected,  no  effects  were  observed  which 
were  as  great  as  0.2  percent  either  way.  The  frequent  references  to  fluores- 
cence in  connection  with  the  effects  observed  seem  therefore  to  be  entirely 
justified. 

The  next  step  undertaken  in  the  investigation  was  a  study  of  the  vari- 
ation of  the  effects  at  anode  and  kathode  with  varying  potential  difference 
at  the  terminals  of  the  cell. 

For  this  purpose  the  original  form  of  cell  was  adopted,  with  electrodes 
0.15  mm.  thick  and  3  mm.  wide,  placed  next  to  the  front  wall  of  the  cell  and 
3  mm.  apart.  The  greater  distance  between  the  electrodes  was  adopted, 
so  that  while  one  of  the  plates  was  being  illuminated  no  stray  light  could 
reach  the  other  electrode.  The  greater  thickness  of  the  electrodes  which, 
in  view  of  the  results  of  previous  work  could  not  materially  affect  the 
sensibility  of  the  cell,  served  to  keep  the  resistance  of  the  cell  as  low  as 
possible. 

The  screen  in  the  front  of  the  light-tight  box  D  was  so  arranged  that  the 
cell  could  be  moved  into  a  position  to  expose  either  electrode  to  the  light 
without  having  to  raise  the  slide  in  the  front  of  the  box  to  make  the  adjust- 
ment. 

As  a  source  of  E.  M.  F.  a  set  of  four  sal-ammoniac  cells  was  first  used,  but 
as  these  were  found  to  drop  slightly  in  voltage  during  a  run,  a  pair  of  storage 
cells  was  substituted  for  them.  The  cells  were  connected  to  the  ends  of  a 
rheocord,  consisting  of  10  meters  of  manganin  wire,  and  sliding  contacts 
were  arranged  so  that  any  desired  potential  difference  could  be  obtained 
at  the  bridge  terminals,  up  to  the  limit  of  the  cells. 

To  the  bridge  terminals  were  also  attached  leads  from  a  large  Weston 
milli-voltmeter,  a  resistance  being  placed  in  series  with  the  meter,  and  the 
whole  calibrated  to  a  total  range  of  15  volts.  With  this  arrangement  it 
was  found  easy  to  obtain  the  potential  difference  between  the  terminals  of 
the  bridge  with  an  accuracy  of  o.oi  of  a  volt,  and  any  given  value  of  the 
potential  difference  could  be  maintained  constant  throughout  the  time 
needed  for  the  readings. 

For  greater  simplicity  in  computation,  and  to  make  it  possible  to  main- 
tain the  potential  difference  between  the  terminals  of  the  cell  at  a  constant 
ratio  to  the  voltmeter  reading,  the  bridge  arms  were  arranged  as  follows 
(see  Fig.isi):  R3  62,500  ohms,  R^  10,000  ohms,  and  RI  a  ioo,ooo-ohm 
box  to  be  used  as  the  variable  resistance.  Thus,  unless  in  a  case  where  the 
resistance  of  the  cell  was  so  great  that  the  io,ooo-ohm  box  had  to  be  varied, 


ELECTRICAL  PROPERTIES  OF  FLUORESCENT  SOLUTIONS. 


159 


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Potential  difference  in  volts  at  cell  terminals 


Fig.  156. 


the  fall  of  potential  across  the  cell  terminals  was  nearly  86  per  cent  of  the 
voltmeter  reading. 

The  change  of  resistance  due  to  the  illumination  was  estimated  in  two 
different  ways,  depending  on  the  magnitude  of  the  effect.  If  the  galvanom- 
eter reading  remained  on  the  scale  when  the  cell  was  illuminated,  a  reading 
of  the  deflection  was  taken.  The  illumination  was  then  stopped  and  a 
second  reading  was  taken  and  the  two  were  averaged.  A  plug  was  then 
inserted  in  the  io,ooo-ohm  box,  the  change  of  galvanometer  reading  obtained, 
then  the  plug  was  removed,  a  second  reading  taken,  and  the  two  averaged. 
Assuming  that  the  galvanometer  deflection  was  proportional  to  the  change 
of  resistance  in  the  io,ooo-ohm  box,  the  number  of  scale  divisions  correspond- 
ing to  a  change  of  one  ohm  or 
one  part  in  ten  thousand  was 
easily  obtained.  As  increase  or 
decrease  in  the  apparent  resis- 
tance of  the  cell  would  produce 
the  same  change  in  the  potential 
difference  at  the  galvanometer 
terminals  as  the  same  percen- 
tage decrease  or  increase  of  the 
resistance  in  R^  it  was  easy 
to  get  quite  satisfactory  esti- 
mates of  the  change  produced 
by  the  illumination.  If  the 
effect  of  the  light  was  great 
enough  to  throw  the  galvanom- 
eter off  the  scale,  a  balance 
was  restored  by  changing  the 
resistance  in  RI.  The  light  was 
then  shut  off  and  a  second 
balance  was  obtained,  and 
from  the  average  of  the  two 
changes  in  RI  the  change  of 
resistance  of  the  cell  was  cal- 
culated. 

Curves  were  plotted,  using 
potential  differences  at  the  cell 
terminals  as  abscissas,  and  Fig-  157- 

change  of  conductivity,  i.  e.,  reciprocals  of  resistance  changes,  as  ordinates. 
Eight  of  these  curves  are  shown  in  the  accompanying  figures.  Each  value 
of  the  conductivity  change  is  the  average  of  at  least  two  readings,  taken 
at  intervals  of  5  minutes  time.  These  curves  are  typical  of  a  large  number 
that  were  obtained  in  a  similar  manner. 

Owing  to  the  variation  in  the  intensity  of  the  arc  light  used,  and  to  the 
difficulty  of  setting  up  the  cells  in  exactly  the  same  manner  each  time,  the 
magnitude  of  the  effects  obtained  varies  considerably  in  different  curves. 
Nevertheless  the  character  of  the  effects  obtained  did  not  vary  greatly. 

In  curves  i  and  2,  3  and  4  (Figs.  156  and  157)  the  kathode  and  anode  were 
illuminated  alternately  throughout  the  experiment.  Each  value  of  the 


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Potential  difference  in  volts  at  cell  terminals. 


160  STUDIES  IX   LUMINESCENCE. 

change  of  conductivity  is  the  average  of  at  least  two  measurements,  made 
at  intervals  of  5  minutes  from  the  end  of  one  measurement  to  the  beginning 
of  the  next.  It  will  be  noticed  that  the  kathode  shows  at  low  voltage  a 
large  decrease  of  resistance  due  to  the  light,  followed  by  a  considerable 
increase  of  resistance  as  the  voltage  rises.  On  the  anode  the  effect  at  low 
voltage  is  just  the  opposite  of  that  on  the  kathode  and  changes  sign  as  the 
voltage  rises  as  in  the  case  of  the  kathode.  It  will  be  noticed  also  that  the 
change  of  sign  of  the  effect  occurs  considerably  earlier  in  the  case  of  the 
anode  than  in  that  of  the  kathode.  The  result  is  that  for  a  considerable 
range  of  voltage  both  electrodes  show  a  decrease  of  resistance  with  the  light. 
As  this  region  happens  to  include  the  voltage  obtained  when  two  gravity 
cells  were  used  as  the  source  of  current,  it  is  not  surprising  that  a  large 
decrease  of  resistance  was  observed  in  the  first  experiments,  when  both 
electrodes  were  exposed  to  the  light  at  once. 

As  the  curves  showed  that  the  effects  on  the  two  electrodes  were  in  gen- 
eral of  opposite  sign,  it  was  thought  advisable  to  arrange  a  cell  so  that  one 
electrode  could  be  studied  at  considerable  length,  and  with  a  certainty  that 
no  complications  would  be  introduced  by  stray  light  reaching  the  other 
electrode. 

In  order  to  have  the  electrodes  as  far  apart  as  possible,  and  still  to  keep 
the  resistance  of  the  cell  within  reasonable  limits,  the  electrode  to  be  exposed 
was  alone  fixed  in  the  front  of  the  cell,  clamped  to  the  front  wall  in  front 
of  and  very  near  the  edge  of  a  narrow  strip  of  glass.  The  other  electrode 
was  made  L-shaped,  nearly  a  centimeter  being  the  length  of  each  leg  of  the 
L  cross-section.  This  electrode  was  placed  in  the  main  part  of  the  cell  with 
the  base  of  the  L  toward  the  side  of  the  cell  away  from  the  light  source. 
This  arrangement  insured  a  very  large  electrode  surface  and  also  an  average 
cross-section  of  the  liquid  between  the  electrodes  sufficient  to  make  up  for 
the  greater  distance  between  them.  It  also  served  to  concentrate  the 
greater  part  of  the  resistance  at  the  electrode  to  be  illuminated. 

As  the  kathode  effect  seemed  the  most  promising  for  further  study  the 
current  was  sent  through  the  cell  from  the  large  electrode  to  the  one  to  be 
exposed. 

A  number  of  runs  were  made  with  this  cell  and  from  the  results  curves 
were  plotted,  of  which  5,  6,  7,  and  8  are  typical  specimens  (Figs.  158,  159, 
and  1 60).  It  will  be  seen  that  the  effects  are  considerably  greater  than 
those  obtained  in  the  former  experiments,  and  also  that  the  form  of  curve 
obtained  is  in  general  more  regular  than  the  others.  The  peculiar  rise  of 
the  effect  after  the  large  decrease  shown  in  the  first  part  of  the  curve  does 
not  appear  to  be  accidental,  but  is  common  to  almost  all  the  curves  plotted 
for  the  kathode  effect. 

Curves  7  and  8  were  made  to  ascertain  the  effect  which  the  previous 
history  of  the  cell  might  have  on  the  action  of  the  light.  Curve  7  was  ob- 
tained in  the  usual  way,  and  a  run  was  made  immediately  after  without 
changing  the  cell,  but  beginning  with  high  voltage  and  reducing  the  voltage 
by  the  same  steps  that  had  been  used  in  going  up. 

It  will  be  noticed  that  the  curves  are  very  much  alike,  the  one  taken  on 
decreasing  voltage  showing  a  slight  shift  toward  the  right.  For  the  last  two 
points  on  the  return  curve  it  was  found  impossible  to  obtain  a  balance  of  the 


ELECTRICAL  PROPERTIES  OF  FLUORESCENT  SOLUTIONS. 


161 


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Potential  difference  in  volts  at  cell  terminals. 
Fig.  158. 


bridge,  the  condition  of  the  cell  appearing  very  unstable.  At  the  three 
voltages  1.03,  1.21,  and  1.38  of  the  first  of  these  two  curves  a  phenomenon 
was  observed  which  occurred  frequently  throughout  the  experiments,  at  the 
point  where  the  effect  reversed.  When  the  cell  was  first  exposed  the  gal- 
vanometer showed  an  imme-  •  ,  

^^  60 

diate   decrease    of    resistance,      "|  50 
followed  in  a  few  seconds,  with      t5  10 
the  light  still  on,  by  a  large 
increase.    When  the  light  was 
shut  off  the  cell  showed  the 
two  changes  in  the  reverse 
order. 

This  would  seem  to  indicate, 
at  least  in  this  region,  a  com- 
bination of  two  effects,  one 
growing  to  a  maximum  more 
rapidly  than  the  other,  and 
dying  away  more  slowly. 

A  few  experiments  were  made 
with  fluorescein  in  absolute 
alcohol,  a  trace  of  caustic  soda 
being  added  to  produce  fluo- 
rescence. The  effects  obtained 
were  very  similar  to  those 
with  the  eosin,  but  not  nearly 
as  marked. 

Rhodamin  was  also  tried, 
with  somewhat  doubtful  re- 
sults, except  that  the  effects 
with  very  low  voltages  were 
fully  equal  to  those  with  eosin. 

Since  there  seemed  to  be  no 
doubt  that  the  seat  of  the  effect 
was  the  thin  film  of  liquid  in 
front  of  the  electrode,  and  since 
the  current  could  not  be  sup- 
posed to  flow  only  to  and  from 
this  film  without  entering  the 
edge  of  the  electrode,  it  did  not 
seem  possible  to  account  for 
the  changes  in  resistance  ob- 
served, except  by  the  actual 
creation  of  an  electromotive 
force  at  the  surface  of  the  elec- 
trode or  near  it,  by  the  action  of  the  light.     So  long  as  low  voltages  were 
used,  such  an  effect  if  it  did  exist  would  account  for  the  change  of  resistance 
when  either  anode  or  kathode  was  illuminated.     For  suppose  the  E.  M.F. 
produced  to  be  such  as  to  tend  to  make  the  exposed  plate  positive  to  the 
unexposed  one.     Then  if  the  exposed  plate  were  made  the  kathode  with 


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Potential  difference  in  volts  at  cell  terminals. 
Fig.  159- 


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1 62  STUDIES  IN   LUMINESCENCE. 

regard  to  the  outside  E.M.F.,  the  E.M.F.  due  to  illumination  would  act 
with  the  original  E.M.F.,  producing  a  greater  current  through  the  cell 
and  making  its  apparent  resistance,  as  measured  on  the  bridge,  less. 

If,  on  the  contrary,  the  direction  of  the  impressed  E.M.F.  were  reversed, 
the  photo-E.M.F.  would  oppose  the  passage  of  the  current  and  the  apparent 
resistance  of  the  cell  be  increased. 

The  reversal  of  the  effects  when  the  voltage  was  raised  is  not  easy  to 
account  for.  From  a  number  of  tests  of  the  polarization  E.M.F.  of  eosin 
cells,  made  by  comparing  the  E.M.F.  with  that  of  a  standard  cell  by  means 
of  a  condenser,  the  effect  of  maximum  polarization  appears  to  be  a  back 
E.M.F.  of  over  2  volts.  This  value  determines  the  point  where  any  con- 
siderable current  begins  to  traverse  the  cell,  and  hence  it  would  be  most 
natural  to  expect  that  just  here  would  occur  any  marked  change  in  the 
phenomena,  such  as  the  reversal  of  the  illumination  effect.  This,  however, 
does  not  seem  to  be  the  case,  for  it  will  be  noticed  from  the  curves  that  the 
reversal  in  the  case  of  the  kathode  occurs  at  very  nearly  1.7  volts,  while  at 
the  anode  the  voltage  is  much  lower.  That  the  effects  at  high  voltages  are 
connected  in  some  way  with  the  polarization  of  the  cell  seems,  nevertheless, 
most  probable,  though  in  what  manner  has  not  as  yet  been  determined. 

Assuming  that  the  effect  at  low  voltage  is  due,  as  has  been  suggested,  to 
a  photo-E.M.F.  similar  to  those  which  have  frequently  been  observed  in 
cells  containing  silver  salts  and  many  other  electrolytes,  attention  was  next 
turned  to  the  effect  of  light  on  the  cell  when  no  external  E.M.F.  was  applied. 

The  same  type  of  cell  was  used  as  had  been  employed  in  all  of  the  experi- 
ments on  the  kathode  alone.  The  terminals  of  the  cell  were  connected 
directly  to  the  Sullivan  galvanometer  which  had  been  used  with  the  bridge 
in  all  the  previous  experiments. 

Very  considerable  effects  were  produced  by  the  action  of  the  light,  ample 
in  magnitude  to  account  for  the  phenomena  observed  with  the  bridge  at 
low  voltages. 

A  series  of  experiments  was  then  undertaken  to  find  if  in  any  way  this 
photo-electric  effect  was  intimately  connected  with  fluorescence. 

In  the  first  place  tests  were  made  throughout  the  visible  spectrum,  and 
curves  plotted  showing  the  variation  of  the  current  produced  in  the  gal- 
vanometer as  a  function  of  the  wave-length  of  the  exciting  light.  The  effect 
was  found  to  increase  regularly  from  the  edge  of  the  visible  spectrum  in  the 
violet  to  a  pronounced  maximum  just  before  the  absorption  band  was  left 
in  going  toward  the  red,  namely,  just  at  the  infra  edge  of  the  green.  The 
decrease  of  effect  from  this  point  on  toward  the  red  was  very  sudden  indeed, 
and  the  effect  was  entirely  lost  before  the  edge  of  the  visible  spectrum  was 
reached.  Three  other  substances,  namely,  rhodamin,  fluorescein,  and 
naphthalin-roth  were  found  to  give  similar  effects,  and  in  each  one  the 
maximum  results  were  obtained  at  the  infra  edge  of  the  absorption  band. 
In  rhodamin  the  effects  were  found  to  be  enormously  greater  than  with  the 
eosin,  but  with  the  other  two  substances  the  results  were  less  satisfactory. 

With  rhodamin  tests  were  made  by  a  condenser  method  to  test  the  mag- 
nitude of  the  E.M.F.  produced,  and  it  was  found  to  be  as  high  as  0.2  of  a 
volt.  In  eosin  it  is  probable  that  the  maximum  was  not  over  half  that 
amount. 


ELECTRICAL  PROPERTIES  OF  FLUORESCENT  SOLUTIONS.       163 

Dilute  solutions  of  rhodamin  were  also  tried  with  very  good  effects.  The 
maximum  dilution  thus  far  tested  is  that  of  one  part  saturated  solution  to 
ten  of  alcohol. 

With  dilute  solutions  of  eosin,  however,  the  effects  were  greatly  reduced. 
The  extremely  thin  capillary  film  was  found  to  be  unnecessary,  either  in 
eosin  or  rhodamin,  though  the  effects  produced  when  the  electrodes  were 
more  than  0.02  mm.  back  from  the  front  of  the  cell  were  very  small. 

Tests  were  made  of  the  absorbing  power  of  both  of  the  substances  in 
saturated  solution,  and  it  was  found  that  practically  no  light  penetrated 
to  a  depth  of  0.02  mm.  in  the  region  of  the  absorption  band.  Thus,  if  the 
effect  is  produced  at  the  surface  of  the  platinum  plate  itself,  a  film  of  this 
thickness  or  greater  would  completely  screen  the  electrode  from  the  action 
of  the  light. 

The  results  obtained  by  Goldman,  whose  paper  is  referred  to  on  page  151, 
agree  in  general  with  those  of  Dr.  Hodge. 

MR.  HOWE'S1  EXPERIMENTS  ON  FLUORESCENT  ANTHRACENE  VAPOR. 

The  following  is  an  account  of  an  attempt  to  determine  whether  fluores- 
cence has  any  effect  on  the  electrical  conductivity  of  anthracene  vapor. 
The  expectation  of  a  change  in  conductivity  during  fluorescence  is  based 
on  the  theory  that  fluorescence  is  a  dissociation  phenomenon.  Although 
the  results  of  the  experiment  were  negative,  it  being  impossible  to  detect 
any  conductivity  of  the  vapor  either  unilluminated  or  fluorescent,  the 
accuracy  of  the  work  was  such  as  to  set  a  rather  definite  upper  limit  for  any 
effect  that  may  be  present. 

Little  work  has  been  done  on  the  conductivity  of  vapors  as  affected  by 
light.  Henry2  could  find  no  conduction  of  iodine  vapor  due  to  light.  On 
the  other  hand,  J.  J.  Thomson3  states,  without  giving  reference,  that  light 
increases  the  conductivity  of  sodium  vapor.  Wiedemann  and  Schmidt,4  in 
their  paper  in  which  they  propose  the  dissociation  theory,  say  that  this  does 
not  call  for  any  ionization  in  the  case  of  fluorescent  vapors. 

The  desirability  of  anthracene  vapor  for  this  experiment  is  seen  from  the 
following  considerations : 

1 .  The  vapor  is  strongly  fluorescent. 

2.  It  has  banded  absorption  and  fluorescence  spectra5  lying  in  the  region 
320-400  fjL/j.,  light  of  these  wave-lengths  being  transmitted  by  the  glass  used 
in  this  experiment.    Stark,6  in  his  work  on  band  spectra,  has  concluded  that 
fluorescence  and  photo-electric  properties  are  intimately  associated  with 
absorption  in  bands  shaded  toward  the  red,  and  that  the  "carriers"  of  the 
band  spectra  are  the  negative  electrons,  which  are  pulled  out  of  their  normal 
position  by  the  incident  light,  and  upon  their  return  to  the  position  of 
equilibrium  liberate  the  energy  of  the  fluorescent  light. 

3.  If  the  separation  of  the  electron  is  complete,  photo-electric  properties 
would  be  exhibited.     It  is  thus  that  Stark  and  Steubing  explain  the  fluores- 

"See  H.  E.  Howe,  Physical  Review,  xxx,  p.  453.  «Wiedemann  and  Schmidt.  Ann.  der  Phys.,  56, 

2Henry,  Proc.  Camb.  Soc.,  9,  p.  319,  1897.  p.  201.  1895. 

'Thomson,  Cond.  through  Gases,  p.  213.  'Elston,  Astr.  Jour.,  25,  p.  155,  1907. 

•Stark,  Phys.  Zeit..  8,  p.  81,  1907. 


164 


STUDIES   IN   LUMINESCENCE. 


cence  of  solid  anthracene.1  After  investigating  a  number  of  substances, 
they  say:  "It  is  highly  probable  that  for  organic  substances,  at  least,  the 
photo-electric  effect  and  fluorescence  are  intimately  connected  with  each 
other."  Hence  it  would  seem  that  the  vapor  of  anthracene  would  be  as 
likely  as  any  vapor  to  possess  photo-electric  properties. 

APPARATUS. 

The  glass  tube  used  to  contain  the  anthracene  is  shown  in  Fig.  161.  The 
larger  part  of  the  tube  was  15  cm.  long  and  2.5  cm.  in  diameter.  The  elec- 
trodes between  which  the  current  passes  consisted  of  an  outer  aluminum 


^Tyrm^HrYTT^rrf^rrrrTTTf^riminM!^^^^ 

I 


Fig.  161. 

cylinder,  to  which  connection  was  made  by  sealing  a  platinum  wire  through 
the  glass  at  M,  and  an  inner  brass  wire  supported  by  a  quartz  tube  Q  and 
visible  through  the  slit  in  the  outer  electrode. 

The  quartz  tube  served  as  insulation  for  the  brass  wire  electrode.  Further 
protection  was  afforded  by  a  deposit  of  silver  on  the  inside  of  the  glass  tube 
around  the  quartz,  this  silver  layer  extending  around  to  the  outside  of  the 
open  end,  where  it  was  wrapped  with  tinfoil  and  grounded  to  form  a  guard- 


Fig.  162. 

ring.     Thus  no  charge  could  leak  over  the  glass  surface  from  the  outer 
electrode. 

The  openings  at  the  outer  end  of  the  quartz  tube  were  sealed  with  de 
Khotinsky  cement.  This  would  not  stand  heating,  and  the  furnace  was 
arranged  to  inclose  only  the  large  part  of  the  tube  (see  dotted  outline  in 
Fig.  162).  It  was  necessary  to  have  the  quartz  tube  fit  snugly  in  place  and 
to  provide  a  tight-fitting  cap  C  (Fig.  161)  at  the  inner  end  in  order  to  pre- 
vent the  sublimation  of  the  anthracene  to  the  colder  parts  of  the  tube. 

1  Stark  and  Steubing,  Phys.  Zeit.,  9,  p.  481,  1908.     Also,  Pochettino,  Nuovo  Citn.,  15,  p.  171,  1906. 


ELECTRICAL  PROPERTIES  OF  FLUORESCENT  SOLUTIONS.       165 

The  tube,  after  being  charged  with  anthracene,  was  exhausted  to  a 
pressure  of  a  few  tenths  of  a  millimeter,  sealed  off,  and  heated  electrically. 
Light  from  a  carbon  arc  A  (Fig.  162)  was  converged  by  the  lens  L,  and  could 
be  passed  into  the  furnace  through  a  mica  window  or  could  be  cut  off  by  the 
screen  S.  When  the  light  was  on,  a  bright  cone  of  violet  fluorescence  could 
be  seen  in  the  center  of  the  tube. 

The  outer  electrode  was  charged  from  a  storage  battery  B,  and  the  rate 
at  which  the  inner  electrode  acquired  a  charge  was  measured  by  means  of  a 
sensitive  Dolezalek  electrometer,  which  was  inclosed  in  a  wire  cage  to  pro- 
tect it  from  the  electrostatic  disturbances.  The  end  of  the  anthracene  tube 
projected  through  a  hole  in  this  cage,  making  the  protection  of  the  inner 
electrode  practically  perfect. 

OBSERVATIONS  AND  RESULTS. 

The  electrometer  needle  was  charged  to  a  constant  potential  (in  most  of 
the  work  this  was  60  volts),  one  pair  of  quadrants  was  grounded,  and  the 
other  pair  connected  to  the  wire  electrode.  This  pair  could  be  grounded 
or  insulated  at  will  by  means  of  the  key  K.  When  K  was  raised  the  motion 
of  the  needle  was  noted  by  observing  on  a  ground-glass  scale  the  image  of  a 
lamp  filament  formed  by  the  small  concave  electrometer  mirror. 

If  VQ  be  the  potential  of  the  charging  quadrants;  K  the  deflection  when 
Vg  is  i  volt;  C  the  capacity  of  the  charging  system;  Q  the  charge;  D  the 
deflection;  and  I,  the  current;  then 

D  =  kX  VQ    and    Q  =  CVq 
I  =  dQ/dt  =  CX  dV/dt  =  kXCX  dD/dt 

on  the  assumptions  that  k1  is  the  same  for  the  moving  needle  as  for  the 
needle  at  rest  and  that  C  does  not  change  with  the  position  of  the  needle. 
Such  assumptions  are  allowable  for  slow  motion  and  small  deflections. 

C  was  determined  by  dividing  the  charge  with  a  cylindrical  condenser  of 
30  cm.  capacity,  and  was  found  to  be  45  cm.  or  5  X  io~5  microfarads. 

A  charging  rate  of  i  division  per  second  could  easily  be  observed  if 
definite.  This  would  mean  a  current  1.3X1  o~13  amperes.  The  insulation 
was  such  that  the  rate  of  leak  when  charged  to  50  divisions  deflection  was 
about  10  divisions  per  minute,  or  one-sixth  the  rate  mentioned  as  easily 
measurable  for  charging. 

At  the  low  pressure  used,  anthracene  vaporizes  sufficiently  to  show  fluores- 
cence between  200°  and  250°  C.  "90  per  cent  sublimed"  anthracene  was 
used  without  attempt  at  purification,  as  it  was  easily  obtainable  and  the 
presence  of  impurities  in  the  material  does  not  seem  to  affect  the  fluores- 
cence of  the  vapor.  No  attempt  was  made  to  shield  the  electrodes  from 
the  light  scattered  by  the  end  of  the  tube,  nor  to  use  monochromatic  light. 

In  liquids  there  is  a  photo-electric  effect,  as  has  already  been  shown  in 
this  chapter,  dependent  upon  which  of  the  electrodes  is  illuminated.  The 
possibility  of  the  existence  of  such  an  effect  in  the  vapor  was  not  here 
considered. 

In  most  of  the  work  the  outer  electrode  was  charged  to  120  volts  from 
an  ordinary  storage  battery.  K  was  raised  and  the  rate  of  motion  noted 
for  the  unilluminated  tube.  This  was  repeated  with  the  light  on. 

'When  the  potential  of  the  needle  was  60  volts  the  value  of  k  was  found  to  be  375  mm.  on  a  scale 
one  meter  distant. 


1 66  STUDIES  IN  LUMINESCENCE. 

When  the  tube  was  newly  made  up  the  electrometer  remained  quiet  under 
all  conditions,  except  for  an  occasional  drift  of  not  more  than  i  division  in  5 
seconds.  In  the  earlier  work,  after  a  few  heatings,  the  electrometer  began 
showing  a  tendency  to  rise  rather  quickly  to  a  more  or  less  definite  deflection, 
regardless  of  conditions  of  potential  or  illumination.  This  peculiar  effect 
varied  from  run  to  run  and  became  very  great  in  a  case  or  two  when  the 
cement  with  which  the  tube  was  sealed  cracked  and  admitted  air.  A  brown 
deposit  was  then  left  in  the  tube,  apparently  the  result  of  a  combination  of 
the  oxygen  of  the  air  with  the  anthracene. 

A  possible  explanation  of  this  attainment  of  a  steady  deflection  even 
when  the  outer  electrode  was  earthed  is  that  the  product  of  the  decom- 
position of  the  anthracene  is  deposited  on  the  quartz  tube  separating  the 
silver  guard-ring  from  the  inner  electrode  and  forms  with  these  two  metals 
a  sort  of  voltaic  cell.  This  "cell"  had  a  very  high  temperature  coefficient 
and  in  some  cases  the  E.M.F.  was  so  great  as  to  throw  the  spot  of  light  off 
the  scale. 

After  several  trials  it  was  found  possible,  by  taking  great  care  to  prevent 
the  entrance  of  air,  to  get  almost  entirely  rid  of  this  effect.  The  elec- 
trometer then  behaved  normally. 

No  ordinary  conduction  was  found.  If  the  light  had  any  effect  it  was 
too  small  to  be  detected.  Thinking  that  the  vapor  under  the  action  of  the 
light  might  be  almost  ionized,  an  attempt  was  made  to  help  out  the  ioniza- 
tion  by  exposing  the  vapor  to  the  action  of  a  small  sample  of  radium 
bromide.  The  ionizing  rays  from  this  substance  did  not  pass  through  the 
glass  sufficiently  to  be  of  any  use. 

Higher  potential  was  obtained  by  making  up  a  set  of  small  lead  sul- 
phuric acid  storage  cells  in  test-tubes.  This  battery,  when  put  in  series 
with  the  one  already  at  hand,  gave  from  360  to  540  volts,  depending  upon 
the  condition  of  the  small  cells.  The  sensibility  was  increased  by  charging 
the  electrometer  needle  to  120  volts.  With  the  greater  sensitiveness  and 
the  unsteady  potential  furnished  by  the  cells,  the  electrometer  wandered 
more  or  less  in  the  neighborhood  of  the  zero,  but  still  did  not  indicate  any 
steady  rate  of  charging  such  as  would  have  been  evident  had  there  been 
appreciable  conductivity  of  the  vapor. 

In  the  freshly  exhausted  tube  the  pressure  was  so  low  that  the  application 
of  400  volts  caused  a  continuous  luminous  discharge  through  the  tube,  but 
this  soon  disappeared  as  the  pressure  rose  due  to  the  vaporization  of  the 
anthracene. 

The  number  of  trials  was  great  enough  to  leave  no  doubt  as  to  the  be- 
havior of  the  vapor  under  the  various  conditions  mentioned. 

CONCLUSIONS. 

1 .  The  conductivity  of  anthracene  vapor  at  the  temperature  and  density 
used  in  this  experiment  is  too  small  to  measure  in  the  manner  described. 

2.  The  effect  of  fluorescence,  if  any  exists,  is  too  small  to  be  detected 
by  the  very  delicate  method  employed. 


CHAPTER  XI. 

ON  FLUORESCENCE  ABSORPTION.1 

In  a  paper  published  in  IQO42  we  used  the  term  fluorescence  absorption 
in  referring  to  the  increase  of  the  absorbing  power  of  a  fluorescent  substance 
which  results  from  fluorescence.  Such  an  effect  was  first  observed  by  Burke,3 
who  found  a  considerable  increase  in  the  absorption  of  uranium  glass  when 
the  glass  was  excited  to  fluorescence.  The  present  writers  observed  the 
same  effect  in  solutions  of  fluorescein  and  eosin.  The  increase  in  the  absorp- 
tion appeared  to  be  greater  for  those  wave-lengths  which  corresponded 
to  the  brightest  regions  of  the  fluorescence  spectrum.  Camichel,4  upon 
repeating  these  experiments,  was  unable  to  detect  any  change  of  absorbing 
power  during  fluorescence  either  in  the  uranium  glass  used  by  Burke  or  in 
the  fluorescent  solutions  tested  by  us.  The  question  was  again  attacked  in 
1907  by  Miss  Wick,5  who  made  a  detailed  study  of  the  phenomenon  in  the 
case  of  an  alcoholic  solution  of  resorufin.  Her  results  consistently  showed  an 
increase  in  the  absorbing  power  of  the  solution  during  fluorescence,  and 
were  in  complete  agreement  with  the  results  obtained  by  us  with  fluores- 
cein and  eosin.  More  recently  a  method  of  detecting  the  effect,  if  it  exist, 
has  been  suggested  by  Wood,6  and  a  few  trials  of  the  method  by  him  led  to 
negative  results.  The  most  recent  experimenter  in  this  field  is  Houstoun,7 
whose  very  careful  experiments  also  fail  to  give  any  indication  of  a  change 
in  absorption  due  to  fluorescence.8 

The  results  obtained  by  ourselves,  and  especially  those  obtained  by  Miss 
Wick,  were  so  definite  and  positive  that  until  recently  we  have  been  of  the 
opinion  that  the  failure  of  others  to  observe  the  effect  was  due  to  the  fact 
that  they  had  not  chosen  suitable  conditions  for  the  experiment.  We  were 
led  to  suspect  the  existence  of  some  systematic  error,  however,  by  the 
results  of  a  careful  study  of  the  collimator  slit  of  the  spectrophotometer 
used  in  our  experiments  upon  the  distribution  of  energy  in  fluorescence 
spectra.9  It  was  found  that  the  screw  was  a  very  accurate  one  and  that  the 
opening  of  the  slit  was  very  closely  proportional  to  the  reading  of  the  micro- 
meter screw.  To  test  this  point  the  slit  was  mounted  in  a  lantern  and  the 
enlarged  image  was  measured  for  a  large  number  of  different  settings.  The 
results  are  shown  in  Fig.  163. 

We  then  tested  the  amount  of  light  passing  through  the  slit  at  different 
widths  by  balancing  two  acetylene  flames  against  each  other,  the  adjustment 
being  made  by  varying  the  slit  width  in  one  case  and  by  varying  the  dis- 

1  Nichols  and  Merritt,  Physical  Review,  xxxi,  p.  500,  1910. 
'Nichols  and  Merritt,  Physical  Review,  xix,  p.  397,  1904. 
•Burke,  Philosophical  Transactions,  igia,  p.  87,  1898. 
«Camichel,  Comptes  Rendus.  vol.  140,  p.  139. 
'Frances  G.  Wick,  Physical  Review,  xxiv,  p.  407,  1907. 
«R.  W.  Wood,  Phil.  Mag.,  16,  p.  940,  1908. 

TR.  A.  Houstoun,  Proc.  Royal  Society  of  Edinburgh,  29,  p.  401,  1909. 

8Since  the  work  described  in  this  chapter  was  first  published,  still  another  article  on  the  subject   has 
appeared,  in  which  the  results  were  also  negative. 
"See  Chapter  XII. 

167 


168 


STUDIES  IN  LUMINESCENCE. 


tance  of  the  flame  in  the  other.  The  experiments  were  performed  in  a  dark 
room  with  an  elaborate  system  of  screens  to  prevent  reflections,  and 
several  independent  tests  convinced  us  that  the  inverse-square  law  of  dis- 
tances was  very  exactly  satisfied.  If  the  intensity  is  computed  by  the  law 

of  inverse  squares,  and  if  a 
curve  is  plotted  showing  the 
relation  between  intensity 
and  slit  width,  the  results 
obtained  are  of  the  type  illus- 
trated in  Fig.  164. 

It  will  be  seen  that  for  nar- 
row slits  the  intensity  of  the 
transmitted  light  is  not  pro- 
portional to  the  slit  width. 
When  the  width  exceeds  a 
few  hundredths  of  a  milli- 
meter the  line  becomes 
straight,  so  that  equal  incre- 
ments in  slit  width  corre- 
spond to  equal  increments  in 
intensity.  The  conditions, 
whatever  they  are,  which 
lead  to  the  curve  in  the 


100 


90 


80 


70 


?  &0 

«> 


50 


40 


30 


20 


10 


30      40       50       60      70 

Drum  reading 
Fig.  163. 


80      90       100 


neighborhood  of  the  zero  of  Fig.  164  are  equivalent  in  their  effect  to  a  shift 
in  the  zero  point  of  the  screw  by  about  2  divisions.  The  intensity  trans- 
mitted by  a  slit  50  divisions  wide  is  not  twice  as  great  as  that  transmitted 
by  one  25  divisions  wide,  but  the  ratio  is  in  reality  48  to  23. 


30 


|- 

I" 

0/5 

l'° 
|5  J 


P7 


2.O 


2        *?        tf        8       JO       /2.       /4       16 
SUT  OPEWNGS    IN    HUNDREDTH^  OF  4 

Fig.  164. 

These  experiments  were  not  made  with  the  same  instrument  that  had 
been  used  by  ourselves  and  later  by  Miss  Wick,  but  refer  to  an  exactly 
similar  Lummer-Brodhun  spectrophotometer.  It  seems  probable  that 
this  effect,  due  possibly  to  diffraction  or  to  reflection  from  the  jaws  of  the 


ON  FLUORESCENCE   ABSORPTION. 


169 


slit,  is  common  to  all  instruments  of  this  type.  It  is  clear  that  if  the  effect 
is  disregarded,  indications  of  fluorescence  absorption  may  be  obtained  even 
if  no  such  effect  exists.  The  method  used  by  Miss  Wick  and  by  ourselves 
involved  three  readings,  namely,  the  intensity  of  the  fluorescent  light  alone, 
the  intensity  of  the  light  transmitted  by  the  solution  when  not  excited, 
and  the  combined  intensity  when  light  was  transmitted  through  the  solu- 
tion at  the  same  time  that  the  latter  was  excited  to  fluorescence.  The  sum 
of  the  first  two  readings  being  found  greater  than  the  third,  it  was  assumed 
that  the  transmission  in  the  latter  case  was  less  than  in  the  first.  If,  how- 
ever, each  of  these  readings  of  intensity  had  been  too  great  by  2  divisions, 
as  indicated  by  Fig.  164,  the  result  of  such  procedure  would  be  to  give  an 
apparent  fluorescence  absorption  measured  by  2  divisions. 

While  it  is  difficult  to  see  how  this  source  of  error  alone  could  account  for 
results  of  the  character  obtained  by  Miss  Wick  and  ourselves,  the  detection 
of  one  source  of  systematic  error  made  it  appear  possible  that  other  similar 
errors  might  be  present,  and  led  us  to  take  up  the  study  of  this  question 
anew.  The  result  of  the  numerous  experiments  which  will  be  briefly 
described  in  this  chapter  has  been  to  convince  us  that  the  phenomenon  of 

F.  x 

T,  Exciting  source 

O         r~f71  ^-Plane  glass 

I — n-Ground  glass 


Ground  glass 


Fig.  165. 


Fig.  1 66. 


fluorescence  absorption  either  does  not  exist,  or  that  the  effect  is  so  small 
that  the  methods  thus  far  used  for  its  detection  are  inadequate. 

In  the  first  method  used  an  attempt  was  made  to  obtain  a  photographic 
record  of  the  effect.  The  arrangement  is  shown  in  diagram  in  Fig.  165. 

The  large  square  cell  F\,  containing  a  solution  of  fluorescein,  was  excited 
from  above  by  a  narrow  beam  of  light,  so  that  a  central  layer  ab  was  excited 
while  the  rest  of  the  solution  was  not.  Directly  back  of  this  was  another 
cell  F2,  also  illuminated  from  above,,  so  that  the  narrow  vertical  strip  cd  was 
excited.  The  photographic  plate  P,  suitably  screened  from  all  sources  of 
light  except  the  fluorescence  in  FI  and  F2,  would  be  fogged  nearly  uniformly 
over  its  surface  by  the  light  from  ab  if  this  alone  was  excited,  and  the  fog- 
ging would  also  be  practically  uniform  if  cd  was  excited.  If,  however,  that 
part  of  the  liquid  in  FI  which  is  excited  to  fluorescence  acquires  the  power 
of  absorbing  the  fluorescent  light  more  strongly  than  before,  then  we  should 
expect  a b  to  cast  a  shadow  upon  the  photographic  plate.  While  this 
shadow  could  not  be  expected  to  be  very  sharp  or  very  dense,  the  results 
obtained  by  ourselves  and  Miss  Wick  indicated  that  under  suitable  con- 
ditions it  ought  to  be  clearly  visible  in  the  negative. 


170  STUDIES  IN   LUMINESCENCE- 

Extended  efforts  were  made  to  obtain  such  conditions  of  concentration, 
thickness  of  layer,  intensity  of  excitation,  etc.,  as  would  bring  out  the 
expected  shadow  on  the  plate  P.  Over  100  negatives  were  made  and  many 
of  these  were  under  conditions  which  appeared  to  us  to  correspond  to  those 
under  which  the  spectrophotometer  had  indicated  a  large  fluorescence 
absorption ;  but  in  no  case  was  a  definite  shadow  observable. 

The  next  method  of  testing  the  matter  is  shown  in  diagram  in  Fig.  166. 
Three  cells  containing  a  solution  of  fluorescein,  or  in  some  cases  resorufin, 
were  used  as  shown  in  Fig.  166.  FI  and  Fs  were  excited  by  the  same  source 
so  as  to  eliminate  errors  due  to  variations  in  excitation;  the  source  used 
was  sometimes  a  quartz  mercury  lamp  and  in  other  cases  the  tungsten  lamp. 
Since  the  solution  was  exactly  the  same  in  all  three  cells,  and  since  FI  and 
Fs  were  at  nearly  the  same  distance  from  the  exciting  source,  the  two  colli- 
mator  slits  were  illuminated  with  almost  equal  brightness.  Any  slight 
inequality  was  balanced  by  opening  or  closing  one  of  the  slits.  This  adjust- 
ment being  made,  the  exciting  source  was  extinguished  and  light  was  sent 
through  the  cell  F3  from  a  small  tungsten  lamp  T2.  To  balance  this  illumi- 
nation, light  from  another  tungsten  lamp,  after  passing  through  the  cell  FZ, 
was  reflected  by  a  piece  of  plane  glass  into  the  second  collimator  slit.  The 
balance  was  obtained  by  adjusting  the  distance  of  T2,  which  slid  upon  a 
graduated  photometer  track.  When  this  balance  was  obtained  the  exciting 
source  was  again  started  and  the  field  of  the  spectrophotometer  observed. 
If  the  effect  of  fluorescence  is  to  increase  the  absorbing  power  of  F3  we 
should  expect  that  while  fluorescence  alone  and  transmission  alone  give  a 
perfect  balance,  there  would  be  a  lack  of  balance  when  excitation  and  trans- 
mission occur  simultaneously.  When  satisfactory  conditions  of  steadiness 
were  obtained  no  such  disturbance  of  balance  could  be  observed.  A  sample 
set  of  readings  is  given  in  Table  20.  The  small  positive  result  obtained  in 
this  case  is  smaller  than  the  errors  of  observation.  In  other  cases  the  results 
indicated  a  small  decrease  in  absorbing  power  during  fluorescence.  The 
experiment  was  tried  with  solutions  of  different  concentration  and  different 
intensities  of  excitation.  But  when  satisfactory  conditions  as  regards  stead- 
iness were  obtained  no  disturbance  of  balance  could  be  detected  which  was 
greater  than  the  errors  of  observation. 

TABLE  20. 
Resorufin.     Excited  by  Mercury  Arc. 

Slit  in  front  of  F3  set  for  equality  of  fluorescence  alone.     Readings  51.6,  49.7,  50.7, 

50.7.     Slit  set  at  the  average  50.7. 
Lamp  Ta  set  for  F+T,  i.  e.,  fluorescence  and  transmission  together.      Distances 

=  233,245.     Average  239. 
Lamp  T2  set  for  transmission  alone  (T)  233,  233.     Average  233. 

F+T,  226,  228.     Average  227. 

Slit  set  again  for  equality  of  fluorescence  alone.     Readings  50.7,  50.4,  50.3,  50.7. 
Slit  set  at  the  average  50.3. 

^+7^:235,239.     Average  237. 

T:  243,  245.     Average  244. 
F+T:  226,  237.     Average  232. 

Slit  set  for  F  alone:  50.8,  50.8,  50.7,  50.6.     Average  50.8. 
F+T:  238,  243.     Average  240.5. 

T:  240,  236.     Average  238. 
F+T:  243,  237.     Average  240. 

Average  of  all:  Distance  of  lamp  for  T  alone  =238.3;  for  T+F= 236.0. 
If  this  difference  is  real  it  indicates  that  the  absorption  of  the  solution  during  fluo- 
rescence exceeds  its  absorption  when  unexcited  by  1.7  per  cent. 


ON  FLUORESCENCE  ABSORPTION. 


171 


It  was  suspected  that  the  phenomenon  of  fluorescence  absorption  appa- 
rently demonstrated  by  Miss  Wick  and  ourselves  might  indicate  not  an 
increase  in  the  absorbing  power  of  the  solution  but  rather  a  decrease  in  its 
power  of  fluorescing.  The  results  obtained  by  us  might  be  interpreted 
equally  well  in  either  of  these  two  ways.  The  fact  that  fluorescence  absorp- 
tion seemed  to  be  in  proportion  to  intensity  of  fluorescence  in  different  parts 
of  the  spectrum  lent  strength  to  this  view.  If  the  effect  is  a  diminution  of 
fluorescence  and  not  an  increase  in  absorption  the  failure  of  our  photo- 
graphic tests  could  also  be  explained  in  an  obvious  manner. 

If  the  fluorescence  is  diminished  we  should  expect  the  diminution  to  be 
observable  not  only  along  the  line  of  the  transmitted  light  but  in  other 
directions.  To  test  this  matter  we  set  up  two  large  fluorescent  cells  FI 
and  F2  (Fig.  167)  covered  with  black  paper  except  on  the  sides  toward  the 
exciting  mercury  arc,  and  having  two  openings  Oi  and  02  in  the  bottom 
through  which  the  light  of  the  tungsten  lamps  TI  and  T2,  after  reflection 
from  mirrors,  might  pass  up  into  the  cell.  The  balance  having  been  ob- 
tained with  fluorescence  alone,  the  lamp  TI  was  then  turned  on  and  we 


T, 

o 


Green  glass 

*—-'-"   ll\  Mercury  arc 

Blue  glass 


0E 
A 


Plane  glass 

I2 T? 

MgCO3 


Fig.  167.  Fig.  168. 

tried  to  determine  whether  there  was  any  change  in  the  balance  resulting 
from  extinguishing  TI  and  at  the  same  time  lighting  T2.  The  only  effect 
was  a  slight  one,  opposite  in  sign  to  that  which  would  be  indicated  in  fluores- 
cence absorption,  and  due  undoubtedly  to  a  small  amount  of  stray  light 
entering  the  slit  after  reflection  from  the  walls  of  the  cell. 

It  seemed  to  us  possible  that  the  effect  might  be  analogous  to  the  effect 
of  infra-red  rays  in  suppressing  fluorescence,  and  that  possibly  it  might  be 
produced  not  by  the  visible  rays  which  caused  this  annoyance  through 
stray  reflection,  but  by  the  red  or  infra-red  rays.  We  therefore  interposed 
ruby  glass  in  the  path  of  TI  and  T2,  thus  eliminating  disturbances  due  to 
stray  light.  Under  these  circumstances  no  disturbance  of  balance  resulted 
from  extinguishing  one  lamp  and  lighting  the  other. 

It  seemed  possible  that  while  fluorescence  absorption  might  really  be  the 
result  of  a  diminution  in  the  intensity  of  fluorescence  this  decrease  might 
not  be  the  same  in  all  directions,  but  might  be  greatest  in  the  direction  in 
which  transmitted  light  proceeded.  The  arrangement  of  apparatus  shown 
in  Fig.  1 68  was  intended  to  test  for  the  effect  in  directions  nearly  but  not 
quite  the  same  as  the  direction  of  transmitted  light.  The  light  from  the 
tungsten  lamp  TI  passes  through  the  cell  FI  so  as  not  to  fall  in  the  slit  of  the 


172  STUDIES  IN   LUMINESCENCE. 

instrument,  but  being  nearly  parallel  to  the  axis  of  the  collimator.  The 
small  amount  of  stray  light  from  7\  which  unavoidably  did  reach  the  slit 
was  balanced  by  light  from  the  second  tungsten  lamp  T2  as  shown  in  the 
figure.  The  results  were  entirely  negative.  Thinking  that  the  intense 
green  line  of  the  mercury  arc  might  produce  so  much  effect  in  weakening 
the  fluorescence  in  FI  that  additional  light  from  Ti  would  produce  only  a 
slight  effect,  we  interposed  in  some  cases  blue  glass  as  shown,  and  in 
order  to  diminish  the  excitation  produced  by  the  light  from  Tt  we  some- 
times used  green  glass  as  indicated.  The  results,  however,  were  in  all 
cases  negative. 

In  Fig.  169  is  shown  an  arrangement  for  testing  the  effect  of  transmitted 
light  upon  fluorescence  in  case  the  direction  of  transmission  is  opposite  to  the 
direction  in  which  fluorescence  is  observed.  No  effect  could  be  detected. 

In  Fig.  170  are  shown  the  essential  parts  of  an  apparatus  used  in  applying 
the  method  suggested  by  Wood.1  A  disk,  shown  in  detail  at  the  right  of  the 
figure,  was  arranged  so  that  the  four  sectors  might  either  occupy  the 
position  shown  in  the  upper  diagram,  or  might  be  shifted  with  reference  to 
each  other  so  that  the  inner  openings  and  the  outer  openings  would  be 

F.  X  X 

T,  LJ  Mercury  arc  Source 

O —^—  Plane  glass 


-Screens 

irr' 

Gollimator  ^Ground  glass 


Fig.  169.  Fig.  170. 

alternate.  This  disk  was  mounted  between  the  exciting  source  and  the 
fluorescent  cell  F,  as  shown.  Light  passed  through  the  outer  sectors  to 
excite  fluorescence,  and  light  passing  through  the  inner  sector,  after  reflec- 
tion from  a  block  of  magnesium  carbonate,  passed  through  the  cell  to  the 
collimator  slit.  Having  balanced  the  illumination  against  a  standard 
placed  before  the  second  collimator  for  the  case  where  the  inner  and  outer 
sectors  were  open  alternately,  the  disk  was  then  changed  so  as  to  make  the 
excitation  and  transmission  simultaneous.  The  results  of  several  sets  of 
observations  with  this  apparatus  are  given  in  Table  21.  The  observa- 
tions indicate  a  slight  negative  effect,  that  is,  the  transmission  of  the  solu- 
tion appears  to  be  greater  during  fluorescence.  But  the  effect  indicated  is 
so  small  that  we  are  inclined  to  look  upon  it  as  resulting  from  accidental 
errors. 

A  few  measurements  were  made  by  a  method  essentially  the  same  as  that 
used  by  us  in  1904,  except  that  the  source  of  the  light  transmitted  was  the 
fluorescence  of  a  portion  of  the  solution.  A  dilute  solution  of  fluorescence 
was  contained  in  a  cell  20  cm.  long,  so  arranged  that  either  the  half  nearest 

'R.  W.  Wood,  Philosophical  Magazine,  p.  940,  1908. 


ON   FXUORESCEXCS   ABSORPTION. 


173 


the  slit  or  that  at  a  distance  could  be  screened  from  the  exciting  light,  which 
in  this  case  was  the  mercury  arc.  The  spectrophotometer  was  set  for  a 
wave-length  near  the  crest  of  the  fluorescence  spectrum  and  not  overlapping 
either  of  the  mercury  lines,  so  that  errors  due  to  stray  light  were  excluded. 
Upon  screening  that  part  of  the  cell  lying  nearest  the  slit  the  field  was 
illuminated  by  the  fluorescence  light  of  the  distant  part  of  the  cell  after 
transmission  through  the  rest  of  the  cell  T.  Upon  screening  the  distant 
part  of  the  cell  the  light  received  in  the  instrument  was  due  to  the  fluo- 

TABLE  21. 

The  numbers  in  the  table  give  the  width  of  the  slit  near  F  (Fig.  170)  when  set 
to  a  balance  with  the  standard  in  front  of  the  other  slit. 


Fluorescein. 

Moderate  concentration. 

Wood's  method. 

Resorufin. 

Dilute 

Wood's  method.   T=i.02  F.  X=o.S99M 

X=o.  523ft 

Sectors  open 

Sectors  open 

Sectors  open 

Sectors  open 

alternately. 

at  same  time. 

alternately. 

at  same  time. 

IOO.9 

IOI.  2 

64.6 

66.2 

97-7 

IOO.  2 

65.2 

64.4 

104.2 

Av. 

IOI 

.58 

100.5 

67.3 

65.4 

103.5 

IOI  .2 

Av.     100.78            66.8 

Av. 

65.98 

65.7    Av 

65 

42 

lOO.J 

IO2.O 

66.6 

66.8 

101  .7 

98.8 

66.4 

65.9 

100.0 

IOO-7 

66.5 

66.2 

100.4 

Av. 

IOO 

.65 

98.7 

Av.    100.05 

66.9 

Av. 

66.68 

66.3    Av 

.    66 

30 

Final 

av.  = 

IOI 

.  1  1 

Final 

av.  =  100.41 

Final 

av.  =  66.33 

Final  av. 

=  65 

86 

Fluorescence  absorption=—  o.oi47\ 

Resorufin. 

Dilute. 

Wood's  method.     T=o.i8  F.    X=o.  599^1 

Resorufin. 

Dilute. 

Wood's  method.    T=7.27  F 

.    X  =  o.599/» 

113.4 

I  I  I  .2 

20.6 

19.0 

I  12.2 

1  12.4 

20.  1 

19.4 

114.7 

112.3 

20.  6 

19.8 

Av. 

H3 

•4 

111.9 

Av.    1  1  1.  95 

20.  i 

Av. 

20.35 

20.  o    Av.     19 

78 

II2.4 

112.3 

20.3 

20.  o 

II3.8 

113.7 

19.9 

20.3 

II3.3 

113.4 

20.4 

20.  5 

II4.7 

Av. 

"3 

•7 

112.  8 

Av.     113.05 

20.4 

Av. 

20.25 

20.7    Av.    20 

38 

Final 

av.  = 

"3 

•55 

Final 

av.  =  113.0 

Final 

av.  = 

20.30 

Final  av. 

=  20 

08 

Fluorescence  absorption  =  —  o 

.033^ 

Fluorescence  absorption  =  —  0.012  T. 

rescence  of  the  nearer  part  F.  Upon  removing  both  screens  fluorescence 
and  transmission  occurred  at  the  same  time  C.  If  absorption  is  increased 
by  fluorescence  we  should  have 

F+T>C 
The  readings  in  one  case  are  given  in  Table  22. 


Average 
Zero 


F 

23-7 
23-7 

22  .9 
23-4 

23.42 
2-4 


TABLE  22. 

T 

23.2 

23-4 
23.6 
22.9 


Average  23 .28 
2.4 


41.2 
41.2 

Average  41  .2 
2.4 


F=2l  .02 


38.8 


174  STUDIES  IN  LUMINESCENCE. 

The  observations  contained  in  Table  22  indicate  a  positive  fluorescence 
absorption  of  15  per  cent.  But  if  we  apply  the  additional  zero  correction 
called  for  by  the  calibration  curve  of  Fig.  164  the  difference  between  F-\-T 
and  C  is  reduced  to  0.9  division  or  4.3  per  cent.  In  other  cases  an  equally 
large  negative  result  was  obtained. 

The  results  of  all  of  these  experiments,  which  have  been  repeated  many 
times,  and  performed  with  more  precautions  to  avoid  false  results  than  can 
be  indicated  in  this  brief  account,  has  been  to  convince  us  that  the  previous 
results  of  both  ourselves  and  of  Miss  Wick  are  due  to  some  systematic 
error,  and  that  the  supposed  increase  in  absorption  due  to  fluorescence 
either  does  not  in  reality  exist  or  is  too  small  to  be  detected  by  these  methods. 
We  have  not  been  able  to  determine  the  exact  nature  of  the  error  which 
led  to  our  preceding  results.  The  peculiar  relation  between  slit  opening 
and  intensity  brought  out  in  Fig.  164  will  explain  some  of  the  results  but 
not  all.  Another  source  of  error  which  might  have  been  an  important  one 
is  that  resulting  from  the  neglect  of  the  slit-width  correction,  to  the  impor- 
tance of  which  attention  is  directed  in  Chapter  XII  of  this  memoir. 


CHAPTER  XII. 

THE  DISTRIBUTION  OF  ENERGY  IN  FLUORESCENCE  SPECTRA.1 

The  energy  of  most  continuous  spectra  is  too  feeble  to  permit  of  accurate 
measurements  excepting  in  the  infra-red  and  the  longer  wave-lengths  of 
the  visible  spectrum,  although  we  have  a  few  determinations  of  the  energy 
of  the  visible  spectrum  of  the  acetylene  flame  by  G.  W.  Stewart2  and  by  Cob- 
lentz3  that  extend  beyond  the  green.  The  direct  measurement  of  the  energy 
of  even  the  brightest  of  fluorescence  spectra,  which  are  very  small  in  intensity 
as  compared  with  those  of  our  ordinary  artificial  light  sources,  is  therefore 
impracticable.  We  have  shown,  however,  in  previous  chapters  of  this 
memoir  that  it  is  possible  to  make  quantitative  spectrophotometric  com- 
parisons between  fluorescence  spectra  and  the  spectrum  of  a  standard  such 
as  the  acetylene  flame.  If  the  distribution  of  energy  of  the  source  used  for 
comparison  be  known,  it  is  therefore  easy  to  compute  that  of  the  fluorescence 
spectrum.  We  have  adopted  this  method  in  determining  the  energy  distri- 
bution in  the  fluorescence  spectra  of  fluorescein,  eosin,  and  resorufin,  with 
the  results  recorded  below.  The  experimental  work  naturally  falls  under 
three  heads :  ( i )  The  determination  of  the  energy  distribution  in  the  stand- 
ard source;  (2)  the  spectrophotometric  comparison  of  the  fluorescence 
spectrum  with  the  standard;  (3)  the  measurement  of  the  absorption  of  the 
fluorescent  liquid,  in  order  that  the  observed  curve  of  fluorescence  may 
be  used  to  compute  the  typical  curve. 

DETERMINATION  OF  THE  DISTRIBUTION  OF  ENERGY  IN  THE  SPECTRUM  OF 
THE  COMPARISON  FLAME. 

The  comparison  source  used  in  the  experiments  to  be  described  in  this 
paper  was  an  ordinary  flat  flame  from  an  acetylene  burner,  in  front  of  which, 
at  a  distance  of  1.4  cm.,  was  mounted  a  metal  screen  having  a  circular  hole 
0.6  cm.  in  diameter,  so  as  to  cut  off  the  light  from  all  but  the  brighter 
central  portions  of  the  flame.  This  flame  with  its  diaphragm  was  mounted 
in  a  metal  box  having  a  circular  window  opposite  the  diaphragm.  The 
box  was  fastened  to  a  base  fitted  to  slide  along  a  straight  metal  track.  This 
track  was  mounted  horizontally  in  the  same  vertical  plane  as  the  axis  of  one 
of  the  collimators  of  a  Lummer-Brodhun  spectrophotometer,  and  at  such  a 
height  that  the  axis  of  the  collimator  extended  would  pass  through  the  win- 
dow in  the  box  and  through  the  center  of  the  diaphragm  to  the  flame  itself. 

In  front  of  the  slit  of  the  collimator  was  mounted  a  sheet  of  clear  white 
glass,  the  surface  of  which  had  been  sufficiently  roughened  by  grinding 
with  powered  carborundum,  so  that  at  whatever  distance  the  flame  might 
be  placed  the  contrast  field  of  the  spectrophotometer  would  be  of  uniform 
brightness  throughout.  The  loss  of  light  by  the  interposition  of  the  ground 

'The  contents  of  this  chapter  first  appeared  in  the  Physical  Review,  xxx,  p.  328. 

JG.  W.  Stewart,  Physical  Review,  xvi,  p.  123. 

'W.  W.  Coblentz.  Bulletin  of  the  Bureau  of  Standards,  vn.  p.  243. 

175 


1 76  STUDIES  IN  LUMINESCENCE. 

glass  was  found  to  be  about  40  per  cent.  Its  transmission  throughout  the 
range  of  wave-lengths  used  in  our  measurements  was  not  measurably 
selective. 

To  determine  the  distribution  of  energy  in  the  spectrum  of  the  light 
received  from  the  comparison  flame  after  passing  through  the  ground  glass 
and  the  optical  parts  of  the  spectrophotometer,  this  spectrum  was  carefully 
compared  wave-length  by  wave-length  with  the  light  received  through  the 
other  collimator  of  the  instrument  from  a  black  body  of  known  temperature. 

The  black  body,  Fig.  171,  consisted  of  a  tube  of  Acheson  graphite  about 
50  cm.  long,  of  1.7  cm.  bore  and  4.0  cm.  external  diameter.  In  the  middle 
this  tube  was  turned  down  for  about  20  cm.  until  the  thickness  of  the  walls 
was  reduced  to  about  0.4  cm.  and  the  thin-walled  cylindrical  chamber  thus 
formed  was  heated  by  means  of  an  alternating  electric  current  furnished  by 
a  step-down  transformer,  of  whose  secondary  circuit  it  formed  the  principal 
part. 

The  ends  of  the  cylindrical  body  were  graphite  plugs,  each  with  an  axial 
hole  i  cm.  in  diameter.  Through  one  of  these  passed  a  tube  of  fused  quartz 
containing  a  platinum-rhodium-platinum  thermo- junction  of  wires  which 
had  been  calibrated  at  the  Bureau  of  Standards.  This  junction  received 
radiation  from  the  surrounding  walls  and  could  be  pushed  in  and  out  at 
will  so  as  to  ascertain  the  range  of  temperatures  within  the  black  body. 

Through  the  opening  in  the     ^^^^^^^^  

other  plug  and  through  cor-  ""T*"         g-*-g^  at^ 

responding  openings  in  dia- 
phragms located  nearer  the  Pig  I?I 
ends  of  the  graphite  tube, 

light  from  the  incandescent  tip  of  the  quartz  tube  reached  the  spectro- 
photometer. 

To  reduce  heat  losses  and  prevent  the  too  rapid  oxidation  of  the  graphite, 
the  tube  was  embedded  to  a  depth  of  about  8  cm.  in  a  mass  of  powered 
magnesite,  which  was  contained  in  a  hollow  cylinder  of  magnesium  oxide 
and  asbestos  such  as  is  used  for  the  packing  of  large  steam  pipes. 

When  the  primary  circuit  was  supplied  with  80  amperes  at  1 10  volts  the 
temperature  of  this  improvised  furnace,  as  indicated  by  the  thermo- junction, 
rose  slowly  to  nearly  1500°  C.,  at  which  temperature  it  remained  with  little 
change  for  a  considerable  time.  Temperatures  were  determined  in  the 
usual  way  by  means  of  a  potentiometer  and  cadmium  cell. 

The  arrangement  of  the  apparatus  is  shown  in  Fig.  172,  in  which  R  is  an 
adjustable  resistance,  A  is  an  a.  c.  ammeter,  P  is  the  primary  coil  of  trans- 
former, ,S  is  the  secondary  coil,  D  is  a  Dewar  flask  with  the  cold  junction  in 
ice,  a  and  b  are  the  collimator  slits  of  the  spectrophotometer,  L  is  the 
Lummer-Brodhun  prism  of  the  spectrophotometer,  0  is  the  observing 
telescope  of  the  spectrophotometer,  g  is  the  ground  glass  in  front  of  slit  b. 

The  slits  a  and  b  were  set  once  for  all  to  convenient  widths,  b  being  30 
divisions  =0.6  mm.  in  width,  and  a  0.06  mm.  The  adjustable  diaphragm 
in  0  was  of  the  same  width  as  slit  b. 

In  this  determination  one  observer  made  settings  for  wave-length  and 
watched  the  contrast  fields  of  the  spectrophotometer,  while  another  recorded 
the  positions  of  the  comparison  flame  when,  for  each  region  of  the  spectrum, 


DISTRIBUTION  OF  ENERGY  IN  FLUORESCENCE  SPECTRA. 


177 


equality  had  been  reached.  In  the  meantime  a  third  observer  followed  the 
changes  of  temperature  with  the  potentiometer  and  recorded  the  E.M.F.  of 
the  thermo-junction  for  each  setting  of  the  spectrophotometer.  Readings 
were  begun  when  a  temperature  of  1410°  (absolute)  was  reached.  Sub- 
sequently the  current  was  slightly  reduced  and  further  sets  of  readings  were 
made  throughout  the  spectrum. 


Fig.  172. 

From  these  data  the  distribution  of  energy  in  the  spectrum  of  the 
comparison  flame  was  computed. 
Wien's  equation 

/x=  Ci/.~5e~w 

was  taken  as  giving  the  energy  in  the  region  of  the  spectrum  at  which 

measurements  were  made.     The  accepted  value,  for  an  ideal  black  body, 

of  the  constant  C2  (i.  e.,  C2  =  14,500)  was  assumed  to  be  applicable  to  the 

present  case,  and  the  quantity  C2/\T  was  calculated  from  the  readings  of 

wave-length    and    temperature. 

Since  relative  values  only  were  TABLE  23. 

desired,  the  constant  Ciwas  given 

a  value  convenient  for  purposes 

of  computation. 

The  two  slits  of  the  spectro- 
photometer were  maintained  at  a 
constant  width  throughout,  and 
the  distance  (d)  of  the  comparison 
flame  was  varied  until  the  inten- 
sities of  the  spectra  were  equal. 

The  energy  of  any  given  region  of  wave-length  X  was  therefore  propor- 
tional to  the  ratio  7A/(i/d2)  where  7A  was  the  energy  of  the  corresponding 
region  of  spectrum  of  the  light  from  the  black  body,  computed  as  above. 

Observations  were  made  for  twelve  regions  lying  between  0.477  M  and 
0.656  n,  and  in  the  course  of  the  determination  the  spectrum  was  traversed 


Wave-length. 

/A/(l/</2) 

Wave-length. 

/A/(«A/2) 

0.656 

248.0 

0-534 

58.70 

.628 

189.0 

.521 

48.50 

.604 

149.2 

.508 

42.50 

.583 

I  12.9 

•497 

37-73 

.565 

87.0 

.486 

29-33 

.548 

69.2 

•477       |     21-25 

178 


STUDIES  IN   LUMINESCENCE. 


four  times.  Good  agreement  existed  between  the  various  readings  in  each 
region  with  the  exception  of  the  observations  immediately  following  changes 
made  in  the  resistance  of  the  primary  circuit.  These  were  rejected  and  all 
other  readings  were  used  in  the  computations  and  averaged  for  each  wave- 
length separately.  The  results  are  given  in  Table  23  and  are  shown  graphi- 
cally in  Fig.  173. 


t 


.48 


.50 


.53 


.54 


.60 


.62 


.66/4 


.56  .58 

Fig.  173. 

Distribution  of  energy  in  the  spectrum  of  the  acetylene  flame.  The  points  marked  5  correspond  to  Stewart's 
direct  measurements  with  the  radiometer.  The  points  marked  A  are  derived  from  Angstrom's  measure- 
ments of  the  Hefner  lamp.  The  dotted  line  is  computed  from  Wien's  equation. 

TABLE  24. 

Spectrophotometric  comparison  of  the  acetylene  comparison  flame,  viewed  through 
the  circular  diaphragm  and  ground  glass,  with  flame  of  a  Hefner  standard  lamp. 


X 

CiH2/Hefner. 

CjH2/Hefner. 

0.656^ 

0.916 

0-534J" 

•32 

.628 

.950 

.521 

•43 

.604 

.990 

.508 

•53 

.590 

I  .00 

•495 

.69 

.583 

1.04 

.487 

.83 

.565 

I  .  II,  I.I5 

•477 

.90 

.548 

1.25 

Wien's  equation  has  been  employed,  although  in  quite  a  different  way, 
by  Knut  Angstrom1  for  the  determination  of  the  distribution  of  energy  in 
the  spectrum  of  the  flame  of  the  Hefner  lamp,  and  it  is  of  considerable 
interest  to  compare  his  determination,  which  involved  neither  the  direct 
measurement  of  temperatures  nor  observations  upon  a  black  body,  with 
the  results  of  the  experiment  just  described. 


•Angstrom,  Nova  Acta  Upsaliensis,  in,  vol.  xxn,  1904. 


DISTRIBUTION   OF  ENERGY   IN  FLUORESCENCE   SPECTRA. 


179 


ao 


For  this  purpose  we  made  a  careful  spectrophotometric  comparison  between 
our  acetylene  standard  and  a  Hefner  flame  (see  Table  2 4  and  Fig.  174).  From 
Angstrom's  curve  of  the  distribution  of  energy  in  the  spectrum  of  the  Hefner 
standard  we  then  computed  the  relative  intensities  of  our  comparison  flame 
for  several  regions  lying  between  0.656  n  and  0.483  /JL.  The  points  so  deter- 
mined are  shown  in  Fig.  173  by  crosses  marked  A. 

Several  points  determined  by  Stewart  by  direct  measurement  with  the 
radiometer  are  also  shown  in  Fig.  173,  being  marked  S. 

It  is  a  matter  of  some  interest  to  determine  to  what  extent  the  visible 
radiation  from  the  acetylene  flame  corresponds  to  the  visible  radiation 
from  a  black  body.  We  find  that  the  curve  computed  from  Wien's  equa- 
tion is  practically  identical  with  our  experimental  curve  from  0.56  n  to 
0.65  ju.  But  for  wave-lengths  less  than  0.56  ju  the  curve  based  upon  Wien's 
equation  (shown  by  the  broken  line  in  Fig.  173)  deviates  considerably  from 
that  determined  by  experiment.  The  acetylene  flame  appears  to  possess  a 
band  of  abnormally  high  radiating  power  in  the  region  lying  between  0.55  /z 
and  the  violet  end  of  the 
spectrum. 

Since  the  first  account 
of  our  own  work  appeared 
Coblentz1  has  published 
the  results  of  direct 
measurements  of  the 
spectrum  of  the  acetylene 
flame  with  the  vacuum 
bolometer.  While  Cob- 
lentz's  results  are  in  fair 
agreement  with  ours  in 
the  region  from  0.48  /*  to 
o.  5  6  fjL,  they  differ  very  appreciably  for  the  longer  waves,  lying  between  our 
own  results  and  those  of  Angstrom. 

COMPARISON  OF  THE  FLUORESCENCE  SPECTRA  WITH  THE  SPECTRUM  OF  THE 
STANDARD  ACETYLENE   FLAME. 

Before  making  the  final  spectrophotometric  determinations  from  which 
the  distribution  of  energy  in  the  spectrum  of  various  fluorescence  spectra 
were  to  be  computed,  a  careful  study  was  made  of  the  effect  of  slit-width 
upon  the  form  of  the  observed  curves. 

In  the  ordinary  use  of  the  Lummer-Brodhun  spectrophotometer  slit  a 
(Fig.  172)  would  be  of  constant  width  and  slit  b  would  be  varied.  The 
accuracy  of  the  screw  of  the  latter  was  therefore  tested,  as  already  described 
at  length  in  the  opening  paragraphs  of  Chapter  XI,  by  mounting  the 
slit,  which  had  been  removed  from  the  instrument,  in  the  field  of  a  pro- 
jecting lantern  and  measuring  the  width  of  the  slit-image,  focused  upon  a 
horizontal  millimeter  scale  at  a  distance  of  about  8  meters.  Variations 
from  constancy  of  the  ratio  of  widths  to  micrometer  readings  were  found 
negligible  for  a  range  of  two  turns  of  the  screw.  Studies  of  the  brightness 
of  the  spectrum  obtained  when  this  slit  was  used  showed,  however,  marked 


10 


0.50 


0.54  0.58 

Fig.  174. 


0.63 


0.66/i 


'Bulletin  of  the  Bureau  of  Standards,  vol.  7.  p.  243,  1911. 


l8o  STUDIES  IN   LUMINESCENCE. 

deviations  from  the  expected  proportionality  between  the  slit-width  and 
intensity,  especially  for  widths  of  less  than  o.oi  cm.  (see  Figs.  163  and  164 
in  Chapter  XI). 

It  was  therefore  decided  to  avoid  changes  in  slit  width  by  using  the 
method  of  comparison  used  in  determining  the  distribution  of  energy  in  the 
spectrum  of  the  comparison  flame.  In  this  method  the  two  slits  of  the  spec- 
trophotometer  remain  unchanged  in  width,  and  equality  of  brightness  is 
obtained  for  each  region  of  the  spectrum  by  moving  the  comparison  flame 
along  a  bar  or  track  parallel  to  the  axis  of  the  collimator.  With  the  system 
of  screens  which  we  employed  to  exclude  stray  light,  the  law  of  inverse 

squares  was  found  to  hold 
for  the  entire  range  of  dis- 
,  tances  used  in  our  experi- 

ments. 

The  three  substances 
selected  for  measurements 
were  fluorescein  in  aque- 
ous solution  slightly  alka- 
line, eosin  in  alcohc 
resorufin  in  alcohol. 


L 


KSI 


* —    I  line,  eosin  in  alcohol,  and 


Fig.  175. 

The  solutions  in  each  case 

were  as  dilute  as  was  found  practicable,  so  as  to  reduce  the  correction  for 
absorption  to  a  minimum. 

In  the  determination  of  the  fluorescence  curves  the  solution  was  placed 
in  a  rectangular  cell  of  white  glass  (/,  Fig.  175)  and  was  excited  by  the  light 
from  a  Cooper-Hewitt  mercury  lamp,  C.  H.  The  tube  of  this  lamp  was 
vertical  and  mounted  at  a  distance  of  about  30  cm.  from  the  wall  of  the  cell. 
Only  those  portions  of  the  tube  were  used  which  were  nearly  in  the  same 
horizontal  plane  as  the  cell.  The  beam  of  exciting  light  entered  the  cell/, 
Fig.  175,  in  the  direction  of  the  arrow  at  right  angles  to  the  axis  of  the 
collimator. 

The  cell  was  inclosed  within  a  metal  box  with  black,  matte,  oxidized 
surfaces,  and  having  only  the  broad  rectangular  opening  de  for  the  admission 
of  the  exciting  light  and  a  narrow,  vertical,  slit-like  aperture  opposite  the 
slit  b  through  which  the  fluorescence  was  viewed. 

The  use  of  the  mercury  arc,  with  its  almost  complete  absence  of  light  in 
the  region  occupied  by  the  fluorescence  bands  to  be  measured,  afforded 
further  protection  against  stray  light.  In  the  study  of  the  fluorescein  solu- 
tion the  additional  precaution  was  taken  of  inserting  a  cell  of  ammonio- 
sulphate  of  copper  in  water  between  the  lamp  and  the  fluorescent  liquid, 
thus  cutting  off  the  yellow  and  green  lines  of  the  arc  almost  completely. 
The  mercury  lamp  was  fed  from  a  storage  battery  of  1 20  volts  with  suitable 
resistance  in  series,  and  under  these  conditions  it  furnished  an  exciting  light 
of  unexpected  constancy,  surpassing  in  this  respect  any  other  source  of  suit- 
able character  and  sufficient  intensity  with  which  we  have  had  experience. 

The  arrangement  of  the  apparatus  for  determining  the  fluorescence  spectra 
is  shown  in  Fig.  175.  The  plan  is  similar  to  that  used  in  comparing  the 
acetylene  flame  with  the  black  body;  but  the  photometer  track  carrying 
the  flame  was  mounted  in  line  with  collimator  a,  while  the  fluorescence  cell 


DISTRIBUTION   OF  ENERGY 'IN   FLUORESCENCE   SPECTRA. 


181 


was  placed  in  front  of  slit  b.  In  the  figure,  A  is  the  comparison  flame;  si 
and  s2  are  screens  to  prevent  stray  light  from  entering  slit  a;  f  is  the  cell  of 
fluorescent  liquid,  and  C.  H.  is  the  Cooper-Hewitt  mercury  arc  lamp. 

The  procedure  was  as  follows: 

Slits  a  and  b  were  set  at  equal  widths  of  50  divisions  =0.05  cm.  The 
comparison  flame  was  then  moved  up  to  a  point  on  the  track  just  in  front 
of  the  aperture  in  screen  sz.  The  observing  telescope  was  set  for  that 
region  of  the  spectrum  corresponding  with  the  maximum  of  the  fluorescence 
band  to  be  measured.  The  fluorescent  solution  in  cell  /  was  diluted  until 
its  spectrum  for  that  region  was  slightly  stronger  than  the  corresponding 
region  in  the  spectrum  of  the  comparison  flame.  By  slightly  shifting  the 


.53 


.55 


•61 


-50  .52  .54 

Fig.  177. — Fluorescein. 
The  observed  points  are  marked 


Fig.  176. — Eosin  (to  left)  and  resorufin. 

The  observed  points  are  marked  by  circles. 

by  circles. 

observing  telescope  in  either  direction  two  places  could  now  be  found,  lying 
a  short  distance  from  the  crest  of  the  fluorescence  band,  at  which  the  two 
spectra  were  of  equal  brightness.  The  circle  readings  of  these  positions 
were  noted,  and  the  position  of  the  comparison  flame  was  read  upon  the 
scale  of  the  photometer  track.  The  flame  was  then  moved  to  a  slightly 
greater  distance  from  slit  a,  and  two  new  positions  were  found  for  the  observ- 
ing telescope,  corresponding  to  points  of  equal  brightness  of  the  band 
farther  from  the  crest.  In  this  way  the  entire  band  was  explored  several 
times,  and  from  these  sets  of  readings  the  intensity  of  the  band  at  various 
wave-lengths  was  computed  in  terms  of  the  corresponding  intensities  of  the 
spectrum  of  the  acetylene  flame. 


1  82  STUDIES   IN   LUMINESCENCE. 

From  these  values  the  observed  curves  in  Figs.  176  and  177  were  plotted. 
The  observed  points  are  indicated  by  circles.  The  crosses  and  dots  in  this 
figure  refer  to  corrected  values  referred  to  in  a  subsequent  paragraph.  In 
the  case  of  eosin  the  wave-length  of  the  crest  is  nearly  the  same  as  that  of 
the  green  mercury  line.  Measurements  in  the  region  of  the  crest  were  there- 
fore rendered  uncertain  by  the  presence  of  stray  light  from  the  green  line. 
For  this  reason  all  the  measurements  in  this  region  have  been  discarded. 
Observations  near  the  crest  of  the  fluorescein  curve  were  also  somewhat 
discordant,  so  that  we  do  not  regard  this  part  of  the  curve  as  determined 
with  much  accuracy. 

To  obtain  from  the  observed  curves  the  distribution  of  energy  in  the 
fluorescence  spectra  it  was  necessary  to  make  corrections  for  slit-  width  and 
absorption,  and  to  multiply  the  ordinates  of  each  corrected  curve  by  the 
ordinates  of  the  same  wave-length  in  the  curve  giving  the  distribution  of 
energy  in  the  spectrum  of  the  acetylene  flame. 

THE   CORRECTION  FOR  SUT   WIDTH. 

In  the  spectrophotometric  comparison  of  sources  of  light  having  con- 
tinuous spectra  and  nearly  the  same  luminosity  curves  the  correction  for 
slit  width  disappears  ;  but  in  the  case  of  spectra  consisting  of  narrow  bands 
this  is  far  from  being  the  case.  The  slit-  width  correction  used  in  the  deter- 
mination of  the  energy  curves  of  incandescent  solids  does  not  apply,  partly 
because  it  is  the  luminosity  of  the  rays  rather  than  their  energy  that  is 
important,  and  partly  because  the  distribution  of  luminosity  in  two  sources 
has  to  be  considered.  The  slit  correction  applicable  to  the  Lummer- 
Brodhun  spectrophotometer  may  be  derived  as  follows  :  Let  the  luminosity 
curve  of  the  source  Si,  in  front  of  the  slit  A,  have  the  equation 


when  the  distance  of  the  source  is  such  as  to  give  the  standard  intensity, 
which  we  shall  call  unity.  If  the  distance  is  varied  so  that  the  intensity 
becomes  i,  then  (X  being  the  wave-length) 


The  luminosity  of  the  source  S2  in  front  of  the  slit  B  will  then  be  given 
by  the  equation 


where  r  is  the  ratio  of  the  energy  of  S2  at  the  wave-length  X  to  the  energy 
of  Si  at  the  same  wave-length,  r  is  itself  a  function  of  X  unless  the  two 
sources  are  identical  in  quality. 

Images  of  the  slit  A  are  formed  in  the  focal  plane  of  the  telescope  for  each 
wave-length  of  the  spectrum  of  Si.  If  the  spectrum  is  continuous,  we 
therefore  have  a  series  of  overlapping  images,  forming  a  spectrum  of  greater 
or  less  impurity  according  to  the  width  of  the  slit  A  . 

The  light  reaching  the  eye  from  the  slit  A  will  depend  upon  the  width 
of  A  ,  being  proportional  to  this  width  if  other  conditions  remain  constant 
But  it  will  also  depend  upon  the  width  of  the  aperture  C  at  the  principal 
focus  of  the  telescope,  through  which  aperture  the  light  used  in  making  the 
setting  must  pass.  If  the  spectrophotometer  is  used  without  an  eye-piece, 


P  RN  Q 
!  T 

i 


DISTRIBUTION   OF  ENERGY   IN   FLUORESCENCE   SPECTRA.  183 

so  as  to  obtain  the  benefit  of  the  contrast  field  formed  by  the  Lummer- 
Brodhun  cube,  all  the  light  passing  through  C  is  used  in  illuminating  the 
field,  and  the  color  is  that  resulting  from  mixing  all  the  wave-lengths 
present.  When  the  instrument  is  set  to  a  match  we  therefore  have  equality 
between  the  total  luminosity  of  the  rays  passing  through  C  from  A ,  and  the 
total  luminosity  of  the  rays  passing  through  C  from  B. 

An  expression  for  the  total  luminosity  of  the  rays  from  A  may  be  found 
as  follows: 

Let  MN  (Fig.  1 78)  be  the  aperture  at  the  focus  of  the  telescope.  It  will  be 
convenient  to  express  the  width  2C  of  this  aperture  in  terms  of  wave-length. 
2C  is  therefore  not  a  constant,  even  if  the  actual  width 
of  the  aperture  is  invariable,  but  depends  upon  the 
dispersion  in  the  region  of  the  spectrum  where  the 
observations  are  made.  The  widths  of  the  slits  A  and 
B,  denoted  by  2 a  and  2b,  respectively,  will  also  be 
expressed  in  terms  of  wave-length.  We  shall  consider 
first  the  case  where  a<c.  M'  d'  f>'  /?'/v'Q' 

Let  the  center  of  C  (00'  in  the  diagram)  correspond  pig  I7g 

to  the  wave-length  X.    That  image  of  the  slit  A  which 
is  formed  by  light  of  wave-length  X  will  have  its  center  coincident  with 
the  center  of  the  aperture.     The  image  formed  by  light  of  wave-length 
\-\-x  will  be  displaced  by  the  distance  x,  so  that  its  central  line  falls  at 
RRf.     For  values  of  x  lying  between  +  (c  —  a)  and  —  (c  —  a)  all  of  the  light 
forming  the  image  of  a  will  pass  through  the  aperture.     The  total  lumi- 
nosity reaching  the  eye  from  such  images  will  therefore  be 
2am  f*~^~(c~a)  2am    s*c~a 

diZ    J-(c-a)  di2     J° 

where  d\  is  the  distance  of  the  source  .Si  from  the  slit  and  m  is  a  factor,  de- 
pending upon  the  dispersion  of  the  prism,  such  that  ma  is  proportional  to  the 
width  of  the  slit  in  scale  divisions.  It  is  clear  that  2am /d?  measures  the 
intensity  of  the  light  entering  the  slit,  the  intensity  being  unity  when  the 
source  is  at  unit  distance  and  the  slit  width  one  division. 

For  values  of  x  lying  between  c—a  and  c-\-a  only  a  part  of  the  image  will 
be  transmitted,  the  transmitted  fraction  being 

_x+a— c 

20 

The  total  transmitted  luminosity  from  these  images  that  are  partially 
transmitted  will  therefore  be 

2am 


/•"•    «+£_«      | 
J  c-a    {    2a      2a  ) 


Upon  expanding/  (X+#)  and/  (X  —  x)  we  have 

2/-/  00  +  .  .  . 


For  any  ordinary  case  the  terms  in  higher  powers  of  x  may  be  neglected 


184  STUDIES   IN    LUMINESCENCE 

The  two  integrals  therefore  become 

-*)  \  dx=     f  ~ 

«^ 


and 

f'+ 

Je_a 


2  a         2a  2  a 


where  /  and  /"  are  written  for  /(X)  and  /"(X)  respectively. 

Upon  adding  the  two  integrals  and  introducing  the  factor  am/d\2 
we  have 


~| 

J  Si 


where 


3 

The  form  of  the  expression  may  be  shown  to  be  the  same  when  a<c. 
Similarly  for  that  portion  of  the  field  which  is  illuminated  by  £2 
2mb r 


When  the  two  fields  are  set  to  equality  we  have  therefore 
2ma  f     ~l       2mb  f       ...  c2(r/)"1 

P\2 


Expanding  ^(r/VcV  and  remembering  that  the  second  term  in  the  bracket 
is  in  each  case  small  we  have  : 


2Cr/ 


'f  _k*'J'  _k#"~l 
cr/        2cr  J 


An  approximate  value  of  r,  usually  a  close  approximation,  is  obtained  by 
neglecting  all  terms  after  the  first.  This  approximate  value  having  been 
plotted  as  a  function  of  X,  r'  and  r"  may  be  determined;  and  since/'  and/" 
may  be  obtained  from  the  luminosity  curve  of  the  source  A  ,  the  correction 
terms  in  the  above  expression  can  readily  be  computed. 

In  the  experiments  described  in  this  paper  the  conditions  were  so  chosen 
as  to  make  a  =  b  =  c.  In  this  case  the  expression  for  r  becomes 

=  djr   _a*   r"     2a?r'n 
dSL       3'  '       3rf  J 

If  Ar  is  the  increase  in  r  which  results  from  changing  the  wave-length 
from  X  to  X+2a,  and  if  A/  is  the  corresponding  increase  in/,  we  have  approxi- 
mately : 

,  /'=A//2a 


DISTRIBUTION   OF  ENERGY  IN   FLUORESCENCE   SPECTRA. 


185 


For  purposes  of  graphical  computation  we  may  conveniently  express  the 
approximate  value  of  r"  in  terms  of  the  distance  DC,  Fig.  179.  If  this  dis- 
tance be  called  8r  it  may  be  readily  shown  that 


Fig.  179- 


3'      6r/J 

In  computing  these  correction  terms  the  lumi- 
nosity curve  given  by  Tufts  was  used.1  The 
values  of  Ar  and  5r  were  taken  from  the  curve 
showing  the  relation  between  X  and  the  approxi- 
mate value  of  r. 

The  correction  depending  upon  8r  is  important 
only  when  the  curvature  of  the  r  curve  is  con- 
siderable. In  the  present  instance  this  correction 
is  large  for  regions  near  the  crest  of  the  curve, 
but  insignificant  elsewhere.  The  importance  of  the  correction  is  well  shown 
in  the  curves  for  resorufin  (Fig.  176),  where  the  points  marked  with  crosses 
show  the  curve  after  the  slit  correction  has  been  applied.  In  the  case  of 
eosin  and  fluorescein  the  region  near  the  crest  was  uncertain  for  the  reasons 
already  mentioned  (p),  so  that  the  slit  correction  has  not  been  applied  in 
this  region. 

The  second  correction  term,  depending  upon  the  product  ArA/,  is  negli- 
gible near  the  crest  of  the  luminosity  curve  of  the  source,  where  A/  is  small. 
The  term  is  thus  of  no  significance  in  the  case  of  eosin  and  fluorescein. 
Even  with  resorufin,  where  the  whole  luminescence  spectrum  lies  well  t:>  thz 
infra  side  of  the  crest  of  the  luminosity  curve,  this  correction  is  important 
only  on  the  steep  side  of  the  curve.  The  result  of  applying  the  correction 
is  shown  by  the  points  in  Figs.  176  and  177  that  are  marked  with  crosses. 

THE   CORRECTION  FOR  ABSORPTION.2 

The  fluorescent  light  which  enters  the  slit  of  the  spectrophotometer  in  the 
foregoing  experiments  comes  from  a  layer  of  liquid  in  the  fluorescence  cell 
having  a  thickness  determined  by  the  opening  de,  Fig.  175,  through  which 
the  exciting  light  enters  the  cell.  This  layer,  which  is  to  be  considered  as 
uniformly  fluorescent,  extends  from  x  =  x\  to  x  =  x2,  where  x  is  the  distance 
from  any  point  from  which  fluorescent  light  emanates  to  the  wall  of  the 
cell  in  the  direction  toward  the  slit  of  the  spectrophotometer. 

Let  the  intensity  of  light,  of  any  wave-length  X,  sent  toward  the  slit 
from  the  unit  layer  be  i,  and  from  a  layer  of  thickness  dx,  be  idx.  If  the 
coefficient  of  absorption  of  the  liquid  be  a  the  light  reaching  the  slit  from 
the  layer  dx  is  idxe~ax  and  the  total  intensity  of  light  reaching  the  slit  from 
the  whole  of  the  excited  layer  is 


=  i  CX*e-a*dx  =  - 
J*.  a 


~axl 


'Tufts's  curve  (see  Physical  Review,  xxrv,  p.  4.33)  was  obtained  for  an  incandescent  light.  But  the  dif- 
ference between  the  color  of  the  acetylene  flame  and  that  of  the  glow  lamp  in  the  region  considered  is  so  small 
that  the  error  introduced  is  quite  inappreciable. 

2For  a  full  description  of  the  methods  employed  in  determining  the  absorption  of  such  solutions  see 
Chapter  XIII. 


1 86 


STUDIES  IN   LUMINESCENCE. 


In  the  cell  used  in  these  determinations  x2  was  36.5  mm.  and  Xi  was 
2.5  mm. 

The  coefficient  a  was  determined  by  measuring  the  transmission  of  a  cell 
containing  a  sufficient  thickness  of  the  fluorescent  liquid  to  reduce  the 
intensity  of  light  of  the  wave-length  of  the  middle  of  the  absorption  band 
to  about  40  per  cent.  The  correction  for  the  losses  of  light  in  the  glass  and 
the  solvent  was  determined  by  measuring  transmission  through  the  same 
cell  when  filled  with  absolute  alcohol  (or  in  case  of  fluorescein  with  water). 

The  correction  for  absorption,  like  that  for  slit-width,  is  of  importance 
only  in  certain  regions.  The  form  of  the  fluorescence  curves  after  the 
application  of  both  corrections  is  shown  in  Figs.  176  and  177. 


THE  ENERGY  CURVES  OF  FLUORESCENCE. 

From  the  data  corrected  in  the  manner  indicated  in  the  two  preceding 
sections  curves  for  the  distribution  of  energy  in  the  fluorescence  spectrum  of 
the  three  substances  under  investigation  were  derived  by  multiplying  each 
ordinate  of  the  corrected  curves  of  Figs.  176  and  177  by  the  ordinate  of  same 
wave-length  in  the  energy  curve  of  the  acetylene  standard  (Fig.  173). 

The  resulting  curves  are  given  in  Fig.  180.  They  suggest  by  their  form 
a  close  resemblance  between  black-body  radiation  and  fluorescence,  except- 
ing that  the  range  of  wave-lengths  in  the  latter  case  is  much  smaller. 
Attempts  to  find  some  simply  modified  form  of  Wien's  equation  which  will 
represent  the  results  have  thus  far  been  unsuccessful. 


CHAPTER  XIII. 

THE  SPECIFIC  EXCITING  POWER  OF  THE  DIFFERENT  WAVE- 
LENGTHS OF  THE  VISIBLE  SPECTRUM  IN  THE  CASE  OF  THE 
FLUORESCENCE  OF  EOSIN  AND  RESORUFIN. 

In  the  case  of  either  a  solid,  like  anthracene,  or  of  a  liquid  such  as  one 
of  the  fluorescent  dyes,  fluorescence  may  usually  be  excited  by  light  of  a 
great  variety  of  different  wave-lengths.  In  fluorescent  solutions  there  is 
commonly  a  well-marked  absorption  band  lying  close  to  the  fluorescence 
band  on  the  side  toward  the  violet,  and  light  of  any  wave-length  within  the 
limits  of  this  absorption  band  will  excite  fluorescence.  In  fact  fluorescence 
may  usually  be  excited  by  light  of  much  shorter  wave-length,  so  that  the 
solution  will  be  lighted  up  when  exposed  to  an  ultra-violet  spectrum.  Simple 
inspection,  however,  is  sufficient  to  show  that  the  intensity  of  the  fluores- 
cence excited  by  various  portions  of  a  given  exciting  spectrum  is  widely 
different.  It  is  not  clear  whether  this  variation  is  due  to  the  fact  that 
certain  wave-lengths  are  particularly  effective  in  exciting  fluorescence, 
or  whether  it  results  merely  from  the  fact  that  the  absorbing  power  of  the 
material  varies  for  different  wave-lengths.  It  is  clear  that  light  can  not 
produce  excitation  unless  it  is  absorbed  by  the  fluorescent  solution.  Dif- 
ferences in  absorbing  power  might  therefore  produce  wide  variations  in  the 
apparent  effectiveness  of  different  spectral  regions  in  producing  fluorescence, 
even  if  the  specific  exciting  power,  i.  e.,  the  fluorescence  excited  per  unit  of 
absorbed  energy,  were  in  reality  constant  for  all  wave-lengths. 

The  determination  of  the  relation  between  the  specific  exciting  power  and 
the  wave-length  of  the  exciting  light  is  a  problem  of  some  interest,  whose 
results  possess  also  considerable  significance  on  account  of  their  bearing 
upon  the  theory  of  fluorescence.  The  present  chapter  deals  with  the  deter- 
mination of  this  relation  for  eosin  and  resorufm.1 

The  exciting  light  was  furnished  by  a  Nernst  glower  which  took  the  place 
of  the  slit  of  a  large  spectrometer.  A  narrow  region  in  the  spectrum  thus 
formed  was  used  in  exciting  the  solution  studied,  and  the  intensity  of 
fluorescence  produced  was  measured  by  a  spectrophotometer  as  the  wave- 
length of  the  exciting  band  was  varied. 

The  arrangement  for  exciting  and  observing  fluorescence  will  be  made 
more  clear  by  inspection  of  Fig.  181.  The  light  of  the  Nernst  glower, 
after  passing  through  the  large  spectrometer  before  mentioned,  was  reflected 
directly  upward  by  a  total  reflecting  prism  as  shown  in  the  figure,  and  passed 
through  a  horizontal  slit  5  into  the  cubical  glass  vessel  which  contained 
the  solution  to  be  studied.  The  spectrometer  was  adjusted  so  as  to  bring 
the  spectrum  in  focus  at  the  slit  S.  One  vertical  face  of  the  cubical  cell 
was  covered  with  a  sheet  of  metal  containing  a  slit  5"  as  shown  in  the  figure. 
The  exciting  light  passing  through  the  slit  S  excited  a  narrow  vertical 

'An  account  of  the  experiments  described  in  this  chapter  was  given  in  the  Physical  Review,  xxxi,  p.  376. 
and  p.  381,  1910. 

I87 


i88 


STUDIES   IN   LUMINESCENCE. 


strip  in  the  liquid  to  fluorescence,  and  this  was  observed  through  the  slit  S', 
in  front  of  which  the  collimator  slit  of  the  spectrophotometer  was  placed. 
The  comparison  source  for  the  spectrophotometer,  as  in  various  experi- 
ments described  in  previous  chapters,  was  an  acetylene  flame  sliding  upon 
a  photometer  track  so  that  it  could  be  set  at  different  distances  from  the 
ground  glass  in  front  of  the  slit.  This  procedure  makes  it  possible  to  keep 
the  slit  width  constant  and  therefore  eliminates  the  errors  which  might  be 
introduced  in  measurements  where  the  variation  of  intensity  is  great. 
The  collimator  slit  of  the  spectrophotometer  was  broad  and  the  instrument 
was  set  so  as  to  measure  the  central  part  of  the  fluorescence  band. 

The  Nernst  glower  which  furnished  the  exciting  light  was  attached 
permanently  to  the  collimator  tube  of  the  large  spectrometer.  The  wave- 
length of  the  light  used  for  excitation  was  varied  by  swinging  the  arm 
carrying  the  collimator  and  glower,  the  other  portions  of  the  spectrometer 
remaining  fixed  in  position.  At  frequent  intervals,  corresponding  to  wave- 
lengths of  the  exciting  light  which  differed  only  slightly  from  one  another, 


To  collimator 

of  spectrophotometer 


Exciting    — > 
light 


Fig.  181. 

the  intensity  of  the  fluorescence  excited  was  measured  by  the  spectropho- 
tometer. The  cell  containing  the  fluorescent  solution  was  then  emptied 
and  carefully  cleaned  and  a  piece  of  magnesium  carbonate  was  mounted 
obliquely  in  the  cell  so  as  to  reflect  light  from  the  slit  S  into  the  spectro- 
photometer. The  intensity  and  range  of  wave-lengths  of  the  light  thus 
reflected  were  measured  for  each  of  the  spectrometer  settings  previously 
used.  Finally  the  absorption  of  a  layer  of  the  solution  of  known  thickness 
was  measured  for  some  definite  wave-length  in  the  spectrum.  By  a  separate 
series  of  measurements  the  relative  reflecting  power  of  the  magnesium 
carbonate  was  determined  throughout  the  region  used. 

The  procedure  in  computing  results  was  as  follows:  To  correct  for  the 
unequal  reflecting  power  of  magnesium  carbonate  at  different  wave-lengths 
the  observed  intensity,  I0>  of  the  exciting  light  was  divided  by  the  reflecting 
power  of  magnesium  carbonate  for  that  particular  wave-length.  This 
correction  reached  approximately  10  per  cent  as  its  highest  value.  Using 
the  values  contained  in  Chapter  XII  for  the  distribution  of  energy  in  the 
spectrum  of  the  acetylene  flame,  we  next  determined  the  relative  energy 
of  the  light  used  for  excitation  as  a  function  of  the  wave-length.  It  will 
be  noticed  that  this  procedure  not  only  takes  account  of  the  difference 


SPECIFIC   EXCITING   POWER  O/  DIFFERENT   WAVE-LENGTHS.  189 

in  quality  and  energy  distribution  between  the  Nernst  glower  and  the 
acetylene  flame,  but  also  eliminates  any  errors  which  might  arise  on  account 
of  selective  transmission  or  other  causes  anywhere  in  the  apparatus. 

The  center  of  the  slit  S'  through  which  the  fluorescence  was  absorbed 
was  8.2  mm.  above  the  bottom  of  the  cell  containing  the  solution.  On 
the  average,  therefore,  the  exciting  light  had  passed  through  8.2  mm.  of 
solution  before  reaching  the  region  whose  fluorescence  was  measured  by 
the  spectrophotometer.  The  light  available  at  this  point  to  produce 
excitation  was  therefore  less  than  that  falling  upon  the  face  of  the  cell  in 
the  ratio  of  one  to  e~082a.  It  was  therefore  necessary  to  compute  this 
factor  e~°  •82a  for  each  wave-length  and  for  this  purpose  we  determined  with 
considerable  accuracy  the  coefficient  of  absorption  of  both  eosin  and  reso- 
rufin  for  different  wave-lengths. 

COEFFICIENTS   OF  ABSORPTION. 

The  measurements,  which  were  extended  throughout  the  absorption 
band  for  both  dilute  and  relatively  concentrated  solutions  of  both  sub- 
stances, were  made  by  comparing  the  intensity  of  the  light  transmitted  by 
a  cell  containing  pure  alcohol  with  the  transmission,  for  the  same  wave- 
length, when  the  cell  was  filled  with  the  solution  in  question.  For  the  con- 
centrated solution  two  cells  were  used,  the  thickness  of  the  absorbing  layer 
being  i  cm.  in  one  case  and  3  cm.  in  the  other.  The  dilute  solutions  were 
contained  in  a  cell  which  gave  an  absorbing  layer  29.5  cm.  thick.  The 
source  of  light  for  transmission  was  an  acetylene  flame.  The  comparison 
standard  was  also  an  acetylene  flame,  which  was  mounted  so  as  to  slide  upon 
a  track  as  previously  described. 

The  ratio  of  the  intensity  of  the  light  transmitted  by  the  solution  to  the 
intensity  of  the  light  transmitted  by  the  alcohol  gave  the  percentage  trans- 
mission, from  which,  with  the  thickness  of  the  cell,  the  coefficient  of  absorp- 
tion, a,  could  be  computed,  a  being  defined  by  the  expression 


where  x  is  the  thickness. 

The  results  for  eosin  are  shown  in  Fig.  182  and  for  resorufin  in  Fig.  183. 
Curve  /  in  each  case  gives  the  coefficient  of  absorption  for  the  dilute  solution 
as  a  function  of  the  wave-length,  while  curve  //  gives  the  coefficient  of 
absorption  for  the  concentrated  solution.  In  Fig.  182  the  scale  is  fifty 
times  greater  for  curve  I  than  for  curve  II,  and  in  Fig.  183  the  scale  for 
curve  7  is  ten  times  as  large  as  that  for  curve  //. 

It  will  be  seen  that  the  absorption  curve  has  very  nearly  the  same  form 
for  the  dilute  and  concentrated  solutions.  This  is  especially  true  in  the 
case  of  resorufin,  where  each  little  ripple  on  the  curve  for  the  dilute  solution 
is  reproduced  on  the  curve  for  the  concentrated  solution.  In  Table  25 
will  be  found  the  values  of  a  for  the  two  solutions,  together  with  the  ratio 
of  the  two  values.  It  will  be  seen  that  the  ratio  remains  nearly  constant 
throughout  the  whole  spectrum.  The  variation  from  constancy  is  most 
marked  at  the  two  ends  of  the  spectrum  where  the  liability  to  experimental 
error  is  greatest.  The  results  in  the  case  of  resorufin  appear  to  indicate 
that  the  form  of  the  absorption  curve  is  not  altered  by  concentration 


190 


STUDIES   IN   LUMINESCENCE. 


throughout  the  range  studied  and  are  thus  in  agreement  with  the  results 
of  Miss  Wick.1 

The  appearance  of  the  curve  for  resorufin  suggests  that  the  band  is 


.50       .52       .54       .56       .58A 
Fig.  182. — Eosin. 

Curve  /  shows  the  coefficient  of  absorption  as  a  func- 
tion of  the  wave-length  for  a  dilute  solution. 
Curve  II  shows  the  same  thing  in  the  case  of  a 
concentrated  solution.  (The  vertical  scale  in 
curve  /  is  fifty  times  greater  than  that  of  curve 
//.)  Curve  F  shows  the  distribution  of  energy 
in  the  fluorescence  spectrum  of  fluorescein. 


.50  .54  .58 

Fig.  183. — Resorufin. 

Curve  /,  coefficient  of  absorption  for  dilute  solu- 
tion; curve  //,  coefficient  of  absorption  for 
concentrated  solution.  (The  scale  of  curve  / 
is  ten  times  greater  than  that  of  curve  //.) 
Curve  //  shows  the  energy  distribution  in  the 
fluorescence  spectrum. 


complex  and  that  in  addition  to  the  principal  maximum  at  0.577  M  there 
are  secondary  maxima  of  absorption  at  about  0.558  p  and  0.536  p.  If  the 
absorption  of  resorufin  really  consists  of  three  overlapping  bands,  as  seems 
probable,  our  results  indicate  that  all  three  of  these  bands  are  produced  by 
the  ions. 

TABLE  25. 
Coefficients  of  absorption  for  resorufin. 


X 

O-l 

Dilute. 

aj 
Concentrated. 

Ratio. 
02   :   tt] 

O  .  4Q7u 

0.0078 

.503 
.508 
.  514 

.OIO7 

O.  I  12 
.125 
.  175 

II.7 

.521 
•527 
•534 
•541 
.548 
•557 
.565 
•574 
.583 
.585 

.Ol8o 

.0307 
.0349 
.0372 
.0467 
.0522 
.0674 
.0448 

.232 
.299 

.385 
.414 
.448 
.563 
.637 
.807 

•576 
.  557 

I2.9 

"\2.6" 
II.  8 
12.0 
12.  0 
12.2 
12.  O 
12.8 

•  593 
•  599 
.604 

.OI26 
.  oo  1  7  . 

f- 
.  162 

.084 

I2.9 

.605 

.032 

Frances  G.  Wick,  Physical  Review,  xxiv,  p.  356;  see  also  Chapter  II. 


SPECIFIC   EXCITING   POWER   OF   DIFFERENT   WAVE-LENGTHS. 


IQI 


In  the  case  of  eosin  there  is  some  indication  of  a  secondary  maximum  in 
the  neighborhood  of  0.49  /x.  The  values  of  a  for  the  two  solutions  and  the 
ratios  for  these  two  values  are  given  in  Table  26.  It  will  be  noticed  that 
the  ratio  is  not  nearly  so  constant  as  in  the  case  of  resorufin.  There  is 
perhaps  some  slight  indication  that  the  secondary  band  at  0.49/1  changes 
with  the  concentration  at  a  different  rate  from  the  principal  band,  but  it 
is  doubtful  whether  the  results  are  sufficiently  accurate  to  give  any  certainty 
to  such  a  conclusion.  The  small  values  of  the  ratio  on  the  red  side  of  the 
band  imply  a  slight  shift  toward  the  red  in  the  case  of  the  dilute  solution, 
but  the  region  in  which  these  small  values  in  the  ratio  occur  is  the  region 
where  the  results  are  most  liable  to  error. 

In  each  case  we  have  shown  by  the  curve  F  the  distribution  of  energy 
in  the  fluorescence  spectrum  of  the  substance  in  question.  The  resemblance 
is  noticeable  between  this  curve  of  energy  distribution  and  the  absorption 
curve.  Roughly  speaking  the  one  curve  is  the  image  of  the  other.  In  the 

TABLE  26. 
Coefficients  of  absorption  of  eosin. 


X 

<*i 
Dilute. 

ttj 
Concentrated. 

Ratio. 
02  :  tt! 

0.480^1 

O.OI  l6 

0.356 

30.7 

.491 

.0153 

.540 

35-3 

•497 

.Ol66 

•597 

35-9 

.503 

.0192 

.736 

38.3 

.508 

.0233 

.870 

37.2 

•5'4 

.0328 

1.274 

38.7 

.521 

.0473 

1.765 

37-3 

•527 

•0533 

1.892 

35-6 

•535 

.0444 

I  .422 

32.0 

•541 

.0235 

0.715 

30.3 

.548 

.0083 

.250 

30.1 

absorption  curve  the  side  toward  the  red  is  steep,  while  the  absorption  dies 
away  gradually  toward  the  violet.  In  the  fluorescence  curve  the  intensity 
dies  away  gradually  toward  the  red  and  stops  abruptly  on  the  violet  side. 
When  the  mechanism  of  fluorescence  is  more  fully  understood  the  reason 
for  this  peculiar  relationship  between  the  two  curves,  which  seems  to  be 
a  characteristic  of  the  fluorescent  substances  of  this  class,  will  doubtless 
be  clear. 

COMPUTATION   OF  SPECIFIC  EXCITING  POWER. 

In  the  determination  of  the  specific  exciting  power  for  eosin  the  solution 
used  was  the  same  as  that  employed  in  determining  the  absorption  curve, 
for  the  more  concentrated  of  the  two  solutions,  shown  in  Fig.  182.  The 
value  of  a  for  each  wave-length  could  therefore  be  read  off  at  once  from 
that  curve. 

In  the  case  of  resorufin  a  different  solution  was  used.  Since,  however, 
the  form  of  the  absorption  curve  is  the  same  for  all  concentrations,  it  was 
sufficient  to  determine  the  absorption  for  one  particular  wave-length  and 
then  to  reduce  the  values  read  from  the  curve  for  that  substance  (Fig.  183) 
in  a  constant  ratio. 


192 


STUDIES   IN   LUMINESCENCE. 


n/ 


Having  computed  in  this  way  the  intensity  of  the  rays  which  actually 
reach  the  region  in  the  solution  under  observation,  and  which  are  therefore 
available  for  producing  excitation,  it  was  only  necessary  to  find  the  amount 
of  energy  absorbed  in  each  case  by  multiplying  /Oe~°'82a  by  a.  Finally 
the  absorbed  fluorescence,  divided  by  the  absorbed  energy  which  produced 
it,  gave  the  quantity  sought,  that  is,  the  intensity  of  fluorescence  excited  by 
unit  quantity  of  absorbed  energy  of  the  particular  wave-length  in  question. 
In  the  case  of  eosin  the  observed  data,  as  well  as  the  derived  quantities 

corresponding  to  several 
steps  in  the  computation, 
are  given  in  Table  27.  In 
the  case  of  resorufin  the 
same  thing  is  shown  graphi- 
cally (Fig.  184).  The  ob- 
served points  in  each  case 
are  connected  by  full  lines, 
while  in  the  case  of  com- 
puted values  the  points  are 
connected  by  broken  lines. 
In  curve  I  we  have  the  in- 
tensity of  the  exciting  light 
after  reflection  from  mag- 
nesium carbonate.  Curve 
II  gives  the  intensity  of  the 
light  actually  reaching  that 
part  of  the  solution  opposite  the  slit  Sf.  The  ordinates  of  curve  III  are 
obtained  by  multiplying  each  of  the  ordinates  of  curve  II  by  a.  Curve  /// 
therefore  gives  the  energy  actually  observed  from  the  exciting  light  for  each 
wave-length.  When  the  ordinates  of  curve  IV,  which  shows  the  observed 
fluorescence,  are  divided  by  the  ordinates  of  curve  ///  we  have  the  quan- 
tity desired,  namely,  the  fluorescence  excited  per  unit  of  observed  energy 
(curve  V). 

TABLE  27. — Eosin. 


.56        .57        .58 

Fig.  184. 

Resorufin. 


I 
X 

2 

Intensity 
of  fluores- 
cence. 

3 
Intensity 
of  exciting 
light. 

4 
Reflecting 
power  of 
MgCOa 

5 
7o  corrected 
for 
reflection. 

6 
Energy  in 

C-2H2 

spectrum. 

7 

h 
(Energy.) 

8 
Ioe—"x 

9 
a/oe—  "* 

10 

Specific 
exciting 
power. 

0.479M 

2.2 

8.25 

102 

8.08 

•'5 

93 

7.0 

2.45 

0.9 

.485 

2.9 

8-4 

103 

8.,5 

•38 

1  I  .2 

7.67 

3   53 

0.82 

.491 

4-1 

8.3 

104 

7.98 

•59 

I2.7 

8-4 

4.24 

0.97 

•497 

53 

8-4 

I05 

8.00 

•77 

14.2 

8.65 

5.20 

I  .02 

.501 

6-3 

IO.  1 

108 

9-37 

.90 

I7.8 

10.  I 

6.98 

.90 

.509 

8.6 

8-9 

1  1  1 

8.00 

2.13 

iy.1 

8.12 

7-37 

•  '7 

•5>4 

10.4 

II  .2 

1  1  1 

10.  I 

2.26 

22.8 

8.13 

10.3 

.01 

.521 

I  I  .0 

1  I  .O 

1  1  1 

99 

2.40 

23.8 

5.60 

99 

.  1  1 

.526 

13.0 

12.2 

1  1  1 

I  I  .0 

2.58 

28.4 

5.98 

11.4 

•'4 

•534 

17.0 

12.  0 

no         10.8 

2.90 

3'-3 

9.76 

13-8 

23 

.540 

18.3 

12.5 

109 

1  1  .  5 

3-«4 

36.2 

19.6 

14.7 

.25 

•547 

1  1  .0 

12.2 

1  08 

11.3 

3-4 

38.4 

31.0 

8.07 

•37 

.556 

4-8 

I2.7 

107 

11.9 

3-84 

84-5 

42.7 

3.85 

•25 

SPECIFIC   EXCITING   POWER  OF  DIFFERENT   WAVE-LENGTHS. 


193 


The  results  for  eosin  are  shown  in  Fig.  185  and  for  resorufin  in  Fig.  186. 
In  the  case  of  the  latter  figure  the  values  corresponding  to  those  shown  in 
Fig.  184  are  marked  by  circles.  The  crosses  show  the  values  obtained  by 
another  set  of  observations  which  are  not  here  recorded  in  detail.  The 
individual  points  on  these  curves  show  considerable  erratic  variation.  In 


Fig.  185. — Eosin. 

Intensity  of  fluorescence  excited  per  unit  of  absorbed  energy  of  different  wave-lengths.     Curve  A  gives  the 
coefficient  of  absorption  and  curve  /•'  the  energy  distributed  in  the  fluorescence  spectrum. 

many  cases  these  can  be  traced  back  to  irregularities  in  the  original  data, 
doubtless  due  to  some  experimental  errors.  In  making  computations  we 
have  made  no  effort  to  eliminate  these  errors  by  smoothing  the  curves,  but 
have  taken  each  observation  as  it  stood  and  carried  through  the  work  to 


.S4 


.S£ 


.S8 


Fig.  1 86. — Resorufin. 

Intensity  of  fluorescence  excited  by  different  wave-lengths  per  unit  of  absorbed  energy.  The  circles  and 
crosses  give  the  results  of  two  independent  sets  of  observations.  Curve  A  gives  the  coefficient  of 
absorption  and  curve  F  the  energy  distribution  in  the  fluorescence  spectrum. 

the  final  result.  Smooth  curves  have,  however,  been  drawn  among  the 
computed  points.  The  data  and  results  recorded  here  represent  the  best  of 
a  number  of  series  of  observations.  While  those  not  recorded  gave  more 
irregular  results  than  here  shown  they  all  agreed  in  indicating  the  same 
general  trend  in  the  final  curve. 


194  STUDIES   IN    LUMINESCENCE. 

In  each  of  these  two  figures  also  we  have  plotted  the  curves  for  the  coeffi- 
cient of  absorption  (A)  and  for  the  energy  distribution  in  the  fluorescence 
spectrum  (F).  It  would  seem  reasonable  to  expect  that  the  curve  of  specific 
exciting  power  might  show  some  peculiarities  either  in  the  region  of  maxi- 
mum absorption  or  in  that  of  most  intense  fluorescence.  It  appears,  how- 
ever, that  nothing  of  this  kind  occurs. 

Before  beginning  these  experiments  we  were  of  the  opinion  that  either 
the  specific  exciting  power  would  prove  to  be  constant,  so  that  the  same 
quantity  of  absorbed  energy  would  produce  the  same  fluorescence  regardless 
of  its  wave-length,  or  else  that  the  effectiveness  of  the  exciting  light  would 
prove  to  be  greater  for  the  shorter  wave-lengths.  The  readiness  with  which 
fluorescence  is  excited  in  the  ultra-violet  spectrum  made  the  latter  view 
seem  plausible.  As  it  turns  out,  neither  of  these  views  is  in  accord  with  the 
facts.  We  were  unfortunately  unable  to  extend  the  observations  to  the 
ultra-violet  spectrum.  It  seems  clear,  however,  that  if  we  confine  our 
attention  to  wave-lengths  falling  within  the  range  of  one  absorption  band 
the  light  lying  near  the  red  side  of  the  band  is  more  effective  in  producing 
fluorescence  than  that  lying  on  the  violet  side,  and  the  change  in  the  specific 
exciting  power  as  we  pass  through  the  band  is  steady,  without  any  indica- 
tion of  anything  selective  in  the  neighborhood  of  the  region  of  maximum 
absorption. 


CHAPTER  XIV. 
THE  THEORY  OF  WIEDEMANN  AND  SCHMIDT. 

The  most  comprehensive  theory  of  luminescence  thus  far  proposed  is 
undoubtedly  that  first  suggested  by  E.  Wiedemann1  in  1889,  and  later 
modified  and  extended  by  Wiedemann  and  Schmidt.2  According  to  this 
theory  some  chemical  or  physical  change  is  produced  in  a  luminescent 
substance  during  excitation,  and  the  emission  of  light  is  an  accompaniment 
of  the  more  or  less  gradual  restoration  of  the  modified  substance  to  its 
original  condition.  Thus,  in  the  case  of  phosphorescence,  it  was  suggested 
by  Wiedemann  that  a  portion  of  the  active  material  was  changed  by  the 
exciting  light  from  the  stable  condition  A  into  the  unstable  state  B.  Phos- 
phorescence would  then  result  from  a  gradual  breaking  down  of  the  unstable 
compound.3  Fluorescence  maybe  due  either  to  the  vibrations  setup  during 
the  change  from  A  to  B,  or  to  the  fact  that  the  reaction  B~~>A  proceeds,  with 
emission  of  light,  during  excitation  as  well  as  during  decay.  Thermo- 
luminescence  is  to  be  explained  as  the  result  of  some  chemical  change  pro- 
duced by  heat,  during  the  progress  of  which  the  molecules  are  thrown  into 
such  violent  vibrations  as  to  bring  about  the  emission  of  light.  Lumines- 
cence ceases  in  such  cases  when  the  change  has  been  completed ;  and  some 
outside  stimulus  is  required,  such  as  that  furnished  by  kathode  rays,  to 
restore  the  substance  to  the  sensitive  state.  Not  only  is  this  explanation 
of  thermo-luminescence  a  plausible  one,  but  in  several  instances  evidence 
has  been  found  that  the  assumed  change  really  occurs.  In  the  paper 
already  referred  to,  Wiedemann  and  Schmidt  have  discussed  all  of  the 
various  types  of  luminescence  in  considerable  detail  and  have  shown  that 
the  theory  proposed  by  them  will  account  for  the  phenomena,  at  least 
qualitatively,  in  a  very  satisfactory  way. 

It  has  generally  been  assumed  that  the  change  accompanying  lumines- 
cence is  of  a  chemical  nature.  Various  suggestions  have  been  made  in 
regard  to  the  character  of  the  reactions  produced  by  the  exciting  light  (or 
other  cause),  and  attempts  are  not  lacking  to  trace  a  connection  between 
luminescence  and  chemical  constitution.  It  was  pointed  out,  however, 
by  Wiedemann  and  Schmidt  that  in  many  cases  there  were  reasons  for 
looking  upon  electrolytic  dissociation  as  the  more  probable  cause  of  lumines- 
cence. In  fact,  since  the  electromagnetic  disturbance  that  constitutes 
light  can  get  a  hold  on  the  molecules  of  the  active  material  only  by  exerting 
forces  upon  the  electrical  charges  in  the  molecule,  and  will  always  tend  to 
separate  the  positive  and  negative  parts,  it  appears  probable  that  the  first 
effect  of  the  exciting  light  is  always  to  produce  some  type  of  electrolytic 
dissociation,  and  that  any  chemical  changes  which  may  be  exhibited  are 


'E.  Wiedemann,  "Zur  Mechanik  des  Leuchtens,"  Wied.  Ann.,  37,  p.  177,  1889. 
2E.  Wiedemann.  and  C.  C.  Schmidt,  Wied.  Ann.,  56,  p.  177,  1895. 
'E.  Wiedemann,  /.  c.,  pp.  224-225. 

195 


196  STUDIES   IN   LUMINESCENCE. 

secondary  effects.1  But  although  the  exciting  light  causes  a  separation  of 
the  active  molecule  into  positive  and  negative  parts,  it  appears  unlikely  that 
these  parts  are  the  ions  of  ordinary  electrolysis.  There  are  many  sub- 
stances like  fluorescein  and  eosin  which  fluoresce  only  when  dissociated.  In 
such  cases  fluorescence  can  not  be  due  to  the  recombination  of  ions;  for 
dissociation  and  recombination  are  taking  place  in  an  electrolytic  solution 
all  the  time,  and  if  this  were  the  cause  of  luminescence  we  should  expect 
the  solution  to  glow  continuously  without  the  action  of  any  exciting  light. 
Fluorescence  in  such  cases  must  be  due  to  some  action  upon  the  ions  them- 
selves. Now  it  can  scarcely  be  doubted  that  the  absorption  of  the  exciting 
light  is  the  result  of  resonance  on  the  part  of  the  molecules  or  atoms  of  the 
active  substance;  and  although  the  vibrational  energy  thus  imparted  to 
the  molecules  is  rapidly  transformed  by  collisions  into  translational  energy 
(heat) ,  yet  under  favorable  conditions  the  molecules  might  be  thrown  into 
such  violent  oscillation  as  to  be  torn  apart.  It  seems,  therefore,  that  the 
most  plausible  assumption  to  make  is  that  the  first  effect  of  the  exciting 
light  is  to  produce  such  violent  vibrations  as  to  liberate  one  or  more  electrons 
from  the  molecule;  in  other  words,  to  bring  about  dissociation  similar  to 
that  produced  by  Roentgen  rays.  For  the  sake  of  definiteness  we  shall 
adopt  this  view  in  the  discussion  that  follows. 

Among  the  facts  of  luminescence  that  are  satisfactorily  accounted  for  by 
the  theory  of  Wiedemann  and  Schmidt  is  the  law  that  the  distribution  of 
intensity  and  the  wave-length  of  maximum  intensity  for  each  band  in  a 
luminescence  spectrum  are  independent  of  the  mode  of  excitation.2  Light 
corresponding  to  any  part  of  the  absorption  band  may,  if  sufficiently  intense, 
produce  dissociation ;  and  dissociation  may  also  be  brought  about  by  vari- 
ous other  agencies,  such  as  the  action  of  Roentgen  rays  and  kathode  rays. 
But  the  manner  in  which  recombination  occurs,  and  therefore  the  form  of 
the  luminescence  spectrum,  will  not  depend  in  any  way  upon  the  manner  in 
which  the  dissociation  was  produced. 

The  theory  also  leads  directly  to  the  conclusion  that  the  light  emitted 
during  the  luminescence  of  an  isotropic  substance  is  unpolarized,  whatever 
may  be  the  condition  of  polarization  of  the  exciting  light,  and  this  con- 
clusion is  in  agreement  with  the  experimental  results  in  all  cases  where 
polarization  tests  have  been  applied. 

The  law  of  Stokes  that  the  light  emitted  during  photo-luminescence  is  of 
greater  wave-length  than  the  exciting  light  has  always  proved  a  stumbling- 
block  in  the  development  of  theories  of  luminescence,  and  at  first  glance 
the  difficulty  appears  to  be  as  great  in  the  case  of  the  Wiedemann  and 
Schmidt  theory  as  in  any  of  the  other  theories  proposed.  Why  should  the 
vibrations  that  occur  on  recombination  differ  in  period  from  those  origin- 
ally set  up  by  the  exciting  light?  It  is  indeed  possible  that  the  latter  are 
forced  vibrations,  whose  period  bears  no  simple  relation  to  the  natural 
period  of  the  molecule ;  but  it  is  more  reasonable  to  expect  that  vibrations 
which  bring  about  actual  disintegration  are  produced  by  resonance.  If 

"It  is  to  be  observed  that  this  reasoning  does  not  apply  to  the  case  of  kathode-luminescence,  since  there 
is  no  reason  why  the  kathode-ray  bombardment  should  not  directly  cause  chemical  changes.  The  relatively 
great  chemical  activity  of  kathode-ray  excitation  as  compared  with  excitation  by  light  is  probably  connected 
with  this  essential  difference  in  the  mechanism  of  excitation  in  the  two  cases. 

zThis  law,  first  proposed  by  Lommel  in  the  case  of  photo-luminescence,  has  been  tested  by  the  writers 
for  numerous  cases  of  fluorescence  excited  by  light  of  different  wave-length  (see  Chapter  I)  and  for  several 
cases  of  excitation  by  kathode  rays  and  Roentgen  rays  (see  Chapter  IX). 


THE   THEORY   OF    WIBDEMANN   AND   SCHMIDT.  IQ7 

this  is  true  we  have  to  do  with  the  natural  period  of  the  molecule  in  both 
cases,  and  it  would  seem  that  the  light  emitted  during  luminescence  should 
have  the  same  wave-length  as  the  exciting  light.  In  the  discussion  that 
follows  we  venture  to  make  a  suggestion  which  offers  an  explanation  of  this 
difficulty. 

In  the  case  of  photo-luminescence  in  solids  and  liquids  the  active  mole- 
cule is  always  closely  surrounded  by  other  molecules.  In  general  these 
surrounding  molecules  belong  to  the  solid  or  liquid  solvent  in  which  the 
active  material  is  dissolved.  If  we  have  to  deal  with  the  luminescence  of  a 
pure  substance  (if  such  cases  occur),  the  surrounding  molecules  are  of  the 
same  kind  as  those  which  participate  in  the  luminescence  phenomena. 
But  in  either  case  the  period  of  vibration  will  be  different  from  what  it 
would  be  if  the  vibrating  molecules  were  isolated.  Since  the  change  in 
period  will  be  relatively  great  for  those  molecules  that  are  close  to  their 
neighbors  and  smaller  for  those  that  are  farther  away,  it  is  seen  that  the 
natural  period  will  vary  through  a  considerable  range  as  the  active  mole- 
cules move  about,  and  at  each  instant  there  will  exist  in  the  substance 
molecules  having  all  periods  lying  between  certain  rather  wide  limits.  The 
absorption  spectrum  of  the  substance  therefore  consists  of  bands  rather 
than  of  lines.1 

When  a  molecule  is  dissociated  by  the  action  of  the  exciting  light,  the 
two  parts,  being  electrically  charged,  will  be  more  strongly  attracted  by 
the  molecules  of  the  solvent  than  was  the  original  neutral  molecule .  Recom- 
bination of  the  separated  ions  is  therefore  more  likely  to  occur  when  the 
latter  are  in  the  immediate  neighborhood  of  the  molecules  of  the  solvent, 
i.  e.,  under  conditions  which  make  the  period  of  the  resulting  vibrations 
longer,  on  the  whole,  than  the  period  natural  to  the  active  molecules  before 
dissociation.  Since  recombination  can  occur  under  a  variety  of  conditions 
a  wide  range  of  wave-lengths  will  be  represented  in  the  luminescence 
spectrum;  the  latter  also  will  consist  of  bands  rather  than  of  lines.  But 
on  the  whole  the  wave-length  of  the  light  emitted  during  luminescence  will 
be  longer  than  that  of  the  exciting  light.  The  same  sort  of  thing  will  occur, 
although  in  less  marked  degree,  even  when  the  active  molecule  is  itself 
electrically  charged,  as  in  the  case  of  fluorescein.  In  this  case,  as  well  as 
in  that  first  considered,  it  may  even  be  that  each  of  the  two  parts  into 
which  the  active  molecule  is  dissociated  becomes  a  nucleus  to  which  neutral 
molecules  are  attracted,  so  that  the  ions  become  heavy  aggregations  of 
molecules.  This  is  the  assumption  usually  made  regarding  the  production 
of  ions  in  gases  by  the  action  of  Roentgen  rays,  kathode  rays,  etc.  If  this  is 
the  real  condition  of  affairs,  the  reasons  for  increased  wave-length  in  the 
light  emitted  are  still  more  evident. 

It  will  be  seen  that  this  way  of  looking  at  the  phenomena  of  photo-lumines- 
cence gives  what  might  be  called  a  mechanical  explanation  of  Stokes's  law. 
It  does  not  lead  us  to  expect,  however,  that  Stokes's  law  will  always  be 
exactly  followed.  The  luminescence  spectrum  and  the  absorption  spec- 
trum may  overlap ;  in  fact,  it  is  to  be  anticipated  that  they  will  do  so  in  the 
majority  of  cases.  This  is  in  agreement  with  the  more  recent  experiments 
on  this  subject. 

'Essentially  this  explanation  of  the  broadening  of  spectral  lines  has  been  discussed  in  some  detail  by 
Oalitzin,  Wied.  Ann.,  56,  p.  78,  1895. 


198  STUDIES   IN   LUMINESCENCE. 

Upon  the  basis  of  the  dissociation  theory  of  luminescence  it  is  clear  also 
that  we  should  expect  no  change  in  the  form  of  the  phosphorescence 
spectrum  during  decay.1  The  intensity  of  phosphorescence  will  depend 
at  each  instant  upon  the  number  of  recombinations  that  occur  per  second, 
and  will  therefore  diminish  as  the  number  of  free  ions  becomes  less.  But 
it  seems  probable  that  the  number  of  recombinations  that  occur  under 
such  conditions  as  to  give  light  of  a  certain  wave-length  will  still  remain 
the  same  fraction  of  the  whole  number.  The  case  has  some  resemblance 
to  that  of  the  distribution  of  velocities  among  the  molecules  of  a  gas ;  if  the 
total  number  of  molecules  in  a  given  volume  is  diminished,  the  number 
having  a  given  velocity  will  also  diminish ;  but  this  number  will  still  be  the 
same  fraction  of  the  whole. 

Probably  the  most  serious  objection  to  the  form  of  the  theory  of 
Wiedemann  and  Schmidt  that  is  here  considered  is  the  absence  of  any 
direct  evidence  of  electrolytic  dissociation  during  the  excitation  of  lumines- 
cence. The  experiments  of  Howe,2  although  carried  out  with  apparatus  of 
high  sensibility,  fail  to  show  any  ionization  in  anthracene  vapor  when 
excited  to  fluorescence.  Ionization  of  gases  by  the  direct  action  of  ultra- 
violet light  has  indeed  been  observed  in  numerous  instances,  but  not  under 
conditions  which  indicate  any  connection  with  luminescence.  No  change 
in  electrical  conductivity  during  excitation,  such  as  might  be  expected  to 
result  from  electrolytic  dissociation,  could  be  detected  in  fluorescent  liquids 
by  Cunningham,  Regner,  or  Camichel.  Our  own  experiments  with 
alcoholic  solutions  of  the  fluorescent  dyes  seemed  at  first  to  indicate  an 
increase  in  conductivity  during  fluorescence  in  some  cases  as  great  as  i  per 
cent.  But  the  work  of  Hodge  and  of  Goldman  has  shown  that  these  results 
were  in  reality  due  to  a  change  in  polarization  or  to  the  establishment  of  a 
photo-electric  E.M.F.2 

On  the  other  hand,  Lenard  has  found  indirect  evidence  of  an  increase  in 
the  conducting  power  of  certain  phosphorescent  sulphides  during  lumines- 
cence. The  fact  that  luminescent  substances  usually  exhibit  strong  photo- 
electric activity,  as  was  first  pointed  out  by  Elster  and  Geitel,  also  points 
to  a  close  connection  between  ionization  and  luminescence.  Even  the 
absence  of  increased  conductivity  in  fluorescent  liquids  is  not  so  important 
as  would  at  first  appear,  for  the  solutions  tested  were  those  in  which  the 
property  of  fluorescence  resides  in  the  ion.  If  a  negative  ion  is  dissociated 
by  the  separation  of  an  electron  no  change  in  the  number  of  ions  will  result, 
and  the  only  change  that  could  be  expected  in  the  conductivity  is  that 
which  might  come  from  a  change  in  the  mobility  of  the  ions.  Whether  the 
mobility  would  be  increased  or  decreased  we  have  no  means  of  telling ;  but 
that  the  change  would  not  be  great  seems  reasonably  certain. 

While  the  failure  of  experiments  to  detect  increased  conductivity  in 
solutions  thus  appears  to  be  without  great  significance,  the  fact  that  cells 
containing  fluorescent  liquids  give  an  E.M.F. ,  when  one  electrode  is  illumi- 
nated, which  is  far  greater  than  that  of  other  photo-active  cells,  gives  another 
instance  of  a  close  connection  between  fluorescence  and  electrical  effects. 


'The  hypothesis  of  molecular  groups  developed  in  Chapter  XV  in  discussing  the  form  of  the  decay  curve 
calls  for  a  slight  change  in  the  form  of  the  spectrum.  But  it  seems  probable  that  this  change  is  so  smalt 
as  to  be  difficult  of  detection. 

'See  Chapter  X. 


THE   THEORY  OF  WIED.EMANN  AND   SCHMIDT.  199 

It  is  to  be  remembered  that  the  rays  that  produce  photo-activity  in  such 
cases  are  the  same  that  cause  fluorescence.  Goldman  has  further  shown 
that  the  behavior  of  photo-active  cells  with  fluorescent  liquids  is  such  as  to 
indicate  the  liberation  of  negative  electricity  at  the  illuminated  electrode 
at  a  constant  rate  depending  upon  the  intensity  of  illumination.  Goldman's 
results  are  thus  in  complete  accord  with  the  view  that  fluorescence  is 
accompanied  by  ionization. 

It  appears,  therefore,  that  while  there  is  no  direct  evidence  of  electrolytic 
dissociation  in  fluorescent  solids  and  liquids,  many  phenomena  have  been 
observed  which  make  it  seem  probable  that  such  dissociation  exists.  A 
detailed  study  of  this  very  interesting  class  of  phenomena  is  much  to  be 
desired,  and  will  doubtless  suggest  some  means  by  which  a  very  fundamental 
question  in  the  theory  of  luminescence  may  be  definitely  settled. 

In  the  case  of  gases  the  experimental  results  are  less  favorable  for  the 
dissociation  theory  of  luminescence,  and  in  the  case  of  fluorescence  seem  to 
directly  contradict  it.  Before  accepting  these  results,  however,  as  con- 
clusive evidence  of  the  falsity  of  the  theory,  it  is  desirable  to  consider  the 
nature  of  the  differences  which  the  theory  would  lead  us  to  expect  between 
the  behavior  of  gases  and  liquids. 

It  can  scarcely  be  doubted  that  in  the  case  of  liquids  or  solid  solutions 
the  presence  of  the  solvent  is  favorable  to  the  ionization  of  the  solute.  The 
solvent  acts  as  a  catalytic  agent,  both  for  ordinary  dissociation  and  for  that 
which  we  assume  to  be  produced  by  light.  To  form  a  picture  of  the  process 
by  which  this  catalytic  action  is  effected,  let  us  consider  a  molecule  of  the 
solute  (active  substance  in  the  case  of  luminescent  substances)  which  is 
subjected  to  some  influence  tending  to  separate  it  into  positive  and  negative 
parts.  Such  a  tendency  might  result  from  a  violent  collision  or  from  strong 
resonant  vibrations  set  up  by  light.  As  soon  as  the  separation  begins  it 
will  be  aided  by  the  attraction  of  the  neighboring  neutral  molecules  of  the 
solvent,  and  when  dissociation  has  actually  occurred  the  ions  are  rendered 
more  sluggish  in  their  movement,  and  are  to  some  extent  prevented  from 
recombining,  by  becoming  attached  to  the  solvent  molecules.  If  we 
imagine  the  solvent  removed,  so  that  we  have  to  deal  with  a  gas  instead  of 
a  solution,  it  is  readily  seen  that  the  conditions  are  less  favorable  to  ioniza- 
tion for  two  reasons : 

(1)  The  forces  to  be  overcome  in  effecting  dissociation  are  far  greater, 
since  there  are  no  solvent  molecules  to  partially  counteract  the  mutual 
attraction  of  the  ions ;  and  for  the  same  reason  the  ions  must  be  driven  much 
further  apart  in  order  that  the  separation  may  be  complete; 

(2)  The    tendency  toward    recombination   is   greatly  increased,  both 
because  of  the  greater  effective  attractive  forces  between  the  ions  and 
because  of  their  more  rapid  motion. 

It  is  not  surprising,  therefore,  that  spontaneous  electrolytic  dissociation 
is  very  rarely  observed  in  gases.  HC1  in  dilute  solution  is  a  good  con- 
ductor. But  the  same  amount  of  HC1  in  the  form  of  a  gas,  occupying  the 
same  volume  as  before,  shows  no  conducting  power  except  such  as  requires 
the  most  refined  apparatus  for  its  detection.  Does  it  not  seem  reasonable 
therefore  that  the  ionization  produced  in  a  gas  by  light  should  also  be  very 
minute,  so  that  we  may  hope  to  detect  it  only  under  especially  favorable 


200  STUDIES   IN   LUMINESCENCE. 

conditions,  and  by  means  of  apparatus  even  more  sensitive  than  that  thus 
far  used? 

It  may  be  also  that  some  type  of  dissociation  occurs  in  gases  that  is  inca- 
pable of  producing  a  change  in  electrical  conductivity,  but  nevertheless  as 
effective  as  complete  dissociation  in  producing  fluorescence.  For  the  sake 
of  definiteness  let  is  assume  that  fluorescence  is  due  to  the  vibrations  of  one 
ring  of  the  Saturnian  atom  proposed  by  Sir  J.  J.  Thomson.  We  can  imagine 
this  ring  set  into  such  violent  vibration  by  light  of  suitable  period  that  an 
electron  is  broken  loose.  This  electron  will  later  return  and,  by  setting 
up  vibrations  when  it  reenters  the  ring  formation,  will  cause  fluorescence. 
But  the  separation  of  an  electron  from  such  a  ring  does  not  necessarily 
mean  dissociation  in  the  electrolytic  sense.  In  order  that  complete  dis- 
sociation should  occur  the  electron  must  be  driven  away  to  such  a  dis- 
tance as  to  be  practically  beyond  the  influence  of  the  remainder  of  the 
atom.  The  energy  required  to  accomplish  this  is  probably  far  greater 
than  that  required  to  break  down  a  ring ;  and  complete  dissociation,  which 
alone  can  manifest  itself  by  an  increase  in  electrical  conductivity,  may  be 
of  relatively  rare  occurrence,  even  in  cases  of  brilliant  fluorescence.  It 
may  be  pointed  out  in  passing  that  the  partial  polarization  observed  by 
Wood  in  the  light  emitted  by  fluorescent  sodium  vapor  is  to  be  expected 
if  the  view  outlined  above  regarding  the  nature  of  fluorescence  in  gases  is 
accepted. 

In  the  case  of  phosphorescent  gases  electrolytic  dissociation  is  readily 
detected  so  long  as  phosphorescence  lasts.  In  such  cases  the  dissociation 
appears  to  be  complete.  The  long  duration  of  the  after-glow  in  gases  sug- 
gests that  the  ions  attach  themselves  to  neutral  molecules  and  in  conse- 
quence move  slowly ;  and  the  fact  that  the  spectrum  often  consists  of  bands 
rather  than  lines  is  in  accord  with  this  view.  The  decay  of  phosphores- 
cence in  gases  follows  the  law  that  is  predicted  by  the  dissociation  theory 
in  its  simplest  form,1  and  seems  to  be  almost  entirely  free  from  the  dis- 
turbing influences  present  in  the  case  of  solids. 

While  we  are  by  no  means  justified  in  looking  upon  the  theory  of 
Wiedemann  and  Schmidt  as  finally  established,  it  appears  to  us,  in  the  light 
of  the  preceding  discussion,  that  it  is  by  far  the  most  promising  of  the 
theories  thus  far  suggested,  and  that  it  affords  a  most  useful  guide  in  the 
experimental  study  of  luminescence. 

'C.  C.  Trowbridge,  Physical  Review,  vol.  26,  p.  515,  1908;   vol.  32,  p.  129,  1911. 


CHAPTER  XV. 

THE  PHENOMENA  OF  PHOSPHORESCENCE  CONSIDERED  FROM 
THE  STANDPOINT  OF  THE  DISSOCIATION  THEORY. 

It  is  our  purpose  in  this  chapter  (i)  to  derive  the  law  of  decay  of  phospho- 
rescence which  the  dissociation  theory  would  lead  us  to  expect  under  such 
ideally  simple  conditions  as  seem  to  exist  in  the  case  of  gases;  and  (2)  to 
consider  in  turn  the  various  modifying  factors  which  may  have  an  influence 
on  the  phenomena  of  phosphorescence  in  solids,  and  to  determine  so  far 
as  possible  the  nature  of  this  influence.  Comparison  with  experimental 
results  will  then  make  it  possible  to  form  an  opinion  of  the  relative  impor- 
tance of  the  various  factors  considered. 

It  will  facilitate  the  discussion  if  we  consider  first  the  requirements  which 
a  satisfactory  theory  must  meet.  The  most  important  experimental  results 
in  the  case  of  photo-luminescence  are  briefly  mentioned  below : 

SUMMARY  OF  EXPERIMENTAL  LAWS. 

1.  Stokes 's  Law. 

2.  If  we  isolate  a  single  band  of  the  luminescence  spectrum  it  is  found 

that  the  distribution  of  intensity  throughout  the  band  is  inde- 
pendent of  the  intensity  and  wave-length  of  the  exciting  light. 

3.  The  light  emitted  by  an  isotropic  substance  during  luminescence  is 

unpolarized,  no  matter  what  may  be  the  condition,  as  regards 
polarization,  of  the  exciting  light. 

4.  During  the  decay  of  phosphorescence  each  band  of  the  luminescence 

spectrum  behaves  as  a  unit;  i.  e.,  the  wave-length  of  maximum 
intensity  and  the  relative  distribution  of  intensity  throughout 
the  band  remain  unchanged. 

These  general  laws  were  discussed  in  the  preceding  chapter  and  it  was 
pointed  out  that  they  are  directly  deducible  from  the  dissociation  theory. 

To  these  four  general  laws  must  be  added  the  following  experimental 
facts  connected  with  the  decay  of  phosphorescence : 

5.  Form  of  Decay  Curve.     The  curve  obtained  by  plotting  the  values 

of  7~*  as  ordinates  and  the  corresponding  values  of  /  as  abscissas 
is  a  straight  line  for  small  values  of  / ;  it  changes  to  a  curve  con- 
cave toward  the  axis  of  /  as  /  increases ;  but  for  still  larger  values 
of  /  the  relation  between  7~*  and  /  is  again  linear,  and  remains 
so  until  7  becomes  too  small  to  measure.  The  form  of  the  decay 
curve  is  dependent  on  the  intensity  and  duration  of  excitation, 
the  slant  being  altered  in  each  of  the  straight  parts  by  a  change 
in  either  of  these  two  factors  in  the  excitation. 

6.  Hysteresis.     The  behavior  of  a  phosphorescent  substance  with  a 

given  excitation  depends  upon  its  previous  history.  Some  semi- 
permanent change  is  produced  by  excitation  which  persists  for 
several  hours,  or  even  for  several  days,  after  visible  phosphores- 
cence has  ceased. 


202  STUDIES   IN    LUMINESCENCE. 

7.  Effect  of  Red  and  Infra-red  Rays.  In  the  case  of  certain  substances 
the  semi-permanent  condition  produced  by  excitation  may  be 
destroyed  and  the  material  restored  to  a  standard  state  by  a  brief 
exposure  to  the  red  and  infra-red  rays.  The  effect  of  the  longer 
waves  during  phosphorescence  is  to  accelerate  the  decay.  In 
some  cases  the  first  effect  is  to  increase  the  brightness  of  phos- 
phorescence, this  temporary  effect  being  followed  by  decay  more 
rapid  than  the  normal. 

THE   DECAY   OF   PHOSPHORESCENCE   UNDER   SIMPLE   CONDITIONS. 

According  to  the  form  of  the  Wiedemann  and  Schmidt  theory  that  is 
here  adopted,  the  effect  of  the  exciting  light  is  to  produce  electric  dis- 
sociation of  the  active  substance  and  the  resulting  negative  and  positive 
nuclei  exist  for  a  time  uninfluenced  by  their  mutual  attraction.  The 
vibrations  that  occur  upon  the  recombination  of  the  ions  give  rise  to  phos- 
phorescence. 

The  simplest  hypothesis  regarding  the  law  of  recombination  of  the  ions 
in  a  luminescent  substance  is  that  which  has  been  applied  to  the  case  of 
ionization  in  gases.1  Let  the  number  of  positive  ions  present  per  cubic 
centimeter  at  any  time  /  be  n.  The  number  of  collisions  between  a  positive 
and  a  negative  ion  will  be  proportional  both  to  the  number  of  positive  ions 
and  to  the  number  of  negative  ions;  and  a  certain  fraction  of  these  col- 
lisions will  result  in  recombination.  Since  positive  and  negative  ions  are 
present  in  equal  numbers  we  have 

dn/dt=—an2  i/n  =  c-\-at  where  c=i/«o 

Since  the  intensity  of  phosphorescence  is  proportional  to  the  number  of 
recombinations  per  second 


This  is  one  form  of  the  empirical  expression  originally  proposed  by 
E.  Becquerel  to  express  the  decay  of  long-time  phosphorescence,  and  is 
the  same  law  that  was  derived  on  the  basis  of  entirely  different  theoretical 
assumptions  by  H.  Becquerel.2 

In  comparing  our  experimental  results  with  the  law  just  derived,  it  is 
convenient  to  write  the  above  expression  in  a  different  form,  namely, 

i 

7/: 

where 

i 
a  =  -—     — 

The  decay  of  phosphorescence  in  gases  appears  to  be  strictly  in  accord- 
ance with  this  law.3  Under  certain  special  conditions  as  regards  temper- 
ature, etc.,  the  law  is  very  closely  obeyed  by  zinc  sulphide  and  even  by 
Balmain's  paint.4  But  in  the  great  majority  of  instances  the  phosphores- 

'Rutherford,  Philosophical  Magazine,  vol.  44,  p.  422,  1897. 

2H.  Becquerel,  Comptes  Rendus,  vol.  113,  p.  618,  1891. 

3C.  C.  Trowbridge.  Physical  Review,  vol.  26,  p.  515;  vol.  32,  p.  129. 

4C.  A.  Pierce,  Physical  Review,  vol.  26,  p.  312  and  p.  454,  1908.     See  also  Chapter  VI. 


PHENOMENA   OF  PHOSPHORESCENCE. 


203 


cence  of  solids  decays  in  accordance  with  a  more  complicated  law.  As 
stated  above,  the  curve  obtained  by  plotting  values  of  I~*  as  ordinates 
and  the  corresponding  values  of  t  as  abscissas  is  not  linear  throughout,  as 
the  simple  theory  would  lead  us  to  expect.  The  curve  is  straight,  both 
for  small  values  of  t  and  for  large  values  of  t,  but  shows  a  sharp  curvature 
for  intermediate  values. 

It  has  been  suggested  by  H.  Becquerel1  that  the  form  of  curve  observed 
in  the  case  of  solids  is  to  be  accounted  for  by  assuming  that  the  phosphores- 
cence spectrum  consists  of  two  bands,  each  of  which  obeys  the  simple  law, 
but  having  different  rates  of  decay.  If  a  decay  curve  is  plotted  for  each 
of  the  two  assumed  bands  the  equations  of  the  two  curves  will  be 


and  each  of  the  two  curves  will  be  a  straight  line.  But  if  we  plot 
against  /,  where  /  is  the  observed  total  inten- 
sity (7  =  /i+/2)  the  resulting  curve  will  not 
be  straight  but  will  take  the  form  shown  by 
curve  C,  Fig.  187.  In  this  figure  A  and  B  are 
the  decay  curves  for  the  two  bands  taken 
separately.  In  the  case  represented  in  the 
figure  the  two  bands  are  assumed  to  decay  at 
widely  different  rates,  and  the  band  which 
decays  more  rapidly  (^4)  has  initially  the 
greater  intensity.  It  will  be  seen  that  under 
these  circumstances  the  decay  curve  for  total 
intensity  has  all  the  characteristics  of  the 
curves  determined  by  experiment.  Becquerel 
cites  one  substance  for  which  the  observed 
intensity  can  be  represented  by  the  expression  pig  l8 

1=  I/(fli-)-6i/)2~|- I/(fl2~l~^202  Illustrating  Becquerel's  explanation  of  the 

form  of  the  decay  curve  in  solids. 

with  remarkable  accuracy.     In  the  case  of 

this  particular  substance  Becquerel  also  found  independent  evidence  of  the 

existence  of  two  bands  in  the  phosphorescence  spectrum. 

This  way  of  accounting  for  the  form  of  the  decay  curve  was  also  pro- 
posed by  Pierce  in  discussing  the  decay  of  phosphorescence  in  Emanations- 
pulver,  and  has  been  shown  by  him  to  give  a  very  satisfactory  explanation 
of  the  change  in  the  form  of  the  decay  curve  resulting  from  changes  in 
temperature. 

If  Becquerel's  explanation  of  the  form  of  the  decay  curve  is  correct  we 
should  expect  to  find  a  change  in  the  phosphorescence  spectrum  during 
decay.  For,  referring  to  the  figure,  the  light  given  out  during  the  first 
few  seconds  is  chiefly  due  to  band  A ,  while  that  emitted  during  the  later 
stages  of  decay  is  almost  entirely  due  to  band  B.  Unless  these  two  bands 
are  coincident  some  shift  in  the  wave-length  of  maximum  intensity  is  to 
be  expected.  Our  own  experiments  with  Sidot  blende  show  that  no  meas- 
urable change  occurs  in  the  form  of  the  spectrum,  or  the  position  of  its 
maximum,  during  the  first  10  seconds,  and  the  later  work  of  Pierce  shows 
that  the  wave-length  of  maximum  intensity  is  unchanged  for  at  least  i 


'Becquerel,  /.  c. 


2O4  STUDIES   IN   LUMINESCENCE. 

minute.1  The  results  of  Waggoner  and  Zeller  with  short-time  phosphores- 
cence, although  less  accurate,  point  to  the  same  conclusion. 

In  the  case  of  our  own  observations  on  Si  dot  blende  we  have  expressed 
the  opinion  that  the  curves  obtained  are  to  be  regarded  as  showing  the 
decay  of  the  green  band  alone;  for  although  Sidot  blende  also  possesses 
bands  in  the  blue  and  violet,  these  decay  so  quickly  and  are  of  such  small 
luminosity  that  they  can  scarcely  affect  the  curves  to  any  appreciable 
extent.  In  the  later  work  of  Werner2  special  precautions  were  taken  in 
the  choice  of  a  substance  and  in  the  use  of  color  screens  to  make  certain 
that  one  band  only  was  studied ;  yet  the  curves  obtained  are  of  exactly  the 
same  type  as  those  found  by  other  observers. 

The  work  of  Werner,  even  more  strongly  perhaps  than  the  work  of 
Pierce  and  our  own  work  with  Sidot  blende,  thus  appears  to  discredit  any 
explanation  of  the  form  of  the  decay  curve  that  is  based  upon  the  assump- 
tion of  two  bands  in  the  phosphorescence  spectrum.  It  can  not  be  denied, 
however,  that  two  bands  may  well  be  present,  even  when  special  precautions 
of  this  kind  are  taken,  and  even  when  the  spectrophotometer  shows  no 
indication  of  a  double  band.  The  bands  of  a  phosphorescence  spectrum 
are  ordinarily  so  broad  that  two  bands  lying  close  together  might  readily 
appear  as  one.  If  the  case  is  one  where  the  bands  actually  overlap  it 
would  not  be  possible  to  make  observations  on  one  alone  without  working 
at  the  extreme  edge  of  the  double  band,  where  the  intensity  would  prob- 
ably be  too  small  to  permit  of  accurate  observations.  An  extremely  small 
shift  in  the  maximum  might  also  fail  of  detection  even  by  the  photographic 
method  of  Pierce.  It  does  not  at  present  appear  possible  to  devise  a  method 
of  obtaining  absolutely  conclusive  proof  that  Becquerel's  explanation  is  not 
correct.  Since  the  assumption  of  two  nearly  coincident  bands  offers  so  direct 
an  explanation  of  the  form  of  the  decay  curve,  and  since  it  seems  certain 
that  curves  may  be  plotted  on  the  basis  of  this  assumption  which  deviate 
from  the  observed  curves  by  less  than  the  experimental  errors,  it  must  be 
admitted  that  the  hypothesis  has  much  in  its  favor. 

There  are  several  reasons,  however,  for  looking  upon  this  explanation 
of  the  form  of  the  curve  with  suspicion.  In  the  first  place,  it  does  not 
account  either  for  the  remarkable  changes  produced  in  the  decay  curves 
by  varying  the  intensity  and  duration  of  excitation,  or  for  the  phenomena 
of  hysteresis  that  have  been  observed  in  almost  all  cases  of  long-time  phos- 
phorescence. It  also  leaves  untouched  the  question  of  the  effects  pro- 
duced by  exposure  to  the  longer  waves.  An  even  more  serious  objection 
is  the  fact  that  it  is  necessary  to  assume  the  existence  of  two  nearly  coin- 
cident bands  in  all  cases  of  long-time  phosphorescence ;  for  upon  replotting 
the  decay  curves  obtained  by  different  observers  with  7~*  and  /  as  co- 
ordinates it  is  found  that  the  curves  are  of  the  same  type  for  all  substances 
thus  far  tested.  It  is  hardly  credible  that  this  is  accidental.  Only  two 
explanations  appear  to  be  possible :  either  the  existence  of  two  nearly  coin- 
cident bands  is  an  essential  characteristic  of  substances  showing  long-time 
phosphorescence,  or  else  the  peculiarities  exhibited  in  the  form  of  the  decay 


'This  result  has  recently  been  confirmed  by  H.  E.  Ives  and  M.  Luckiesh  (Astrophysical  Journal,  vol. 
xxxiv,  p.  173,  1911),  who  find  that  the  phosphorescence  spectrum  remains  the  same  for  at  least  fifteen 
minutes  after  the  beginning  of  decay. 

ZA.  Werner,  Ann.  der  Phys.,  24,  p.  164,  1907. 


PHENOMENA    OF   PHOSPHORESCENCE.  205 

curve  are  to  be  explained  in  some  manner  which  does  not  involve  the 
assumption  of  two  bands  at  all.  While  it  may  prove  of  interest  at  some 
later  time  to  develop  the  theory  along  the  lines  suggested  by  the  first  of 
these  alternatives,  the  present  discussion  will  be  based  upon  the  acceptance 
of  the  second,  and  \ve  shall  consider  in  what  ways  the  form  of  the  decay 
curve,  as  well  as  certain  other  peculiarities  of  phosphorescence,  may  be 
explained  in  substances  possessing  only  one  band  in  the  phosphorescence 
spectrum. 

ABSORPTION   EFFECTS. 

If  a  homogeneous  substance  possessing  only  one  band  in  its  phosphores- 
cence spectrum  is  uniformly  excited  throughout,  and  if  the  light  emitted 
by  the  interior  portions  suffers  no  diminution  by  absorption  before  reaching 
the  surface,  then  according  to  the  dissociation  theory  here  considered  the 
decay  of  phosphorescence  will  be  in  accordance  with  the  law 

P 

v/ 

These  conditions,  however,  can  never  be  exactly  attained.  The  exciting 
light  must  be  absorbed  to  some  extent,  for  otherwise  no  energy  would  be 
available  to  produce  phosphorescence.  The  excitation  will  therefore  be 
greatest  at  the  surface,  and  will  diminish,  at  a  rate  determined  by  the 
absorbing  power  of  the  material,  as  we  pass  to  points  within.  Since  the 
ions  are  in  consequence  more  numerous  in  the  surface  layers,  and  since 
the  number  of  recombinations  per  second,  which  determines  the  intensity 
of  phosphorescence,  is  proportional  to  the  square  of  the  number  of  ions, 
it  is  clear  that  the  light  emitted  during  the  early  stages  of  decay  will  come 
chiefly  from  the  surface.  As  decay  proceeds,  however,  the  relatively  high 
rate  of  recombination  at  points  where  n  is  large  will  cause  a  rapid  approach 
to  uniformity  of  ionic  concentration,  and  the  part  contributed  to  the  total 
light  by  the  interior  of  the  mass  will  become  increasingly  important.  At 
first  the  intensity  of  phosphorescence  is  approximately  proportional  to 
Fiaw2,  where  V\  is  the  volume  of  the  surface  layer  that  is  chiefly  effective. 
Later,  when  the  light  from  the  interior  becomes  comparable  in  intensity 
with  that  from  the  surface,  the  phosphorescence  will  be  approximately 
proportional  to  V->a.n*,  where  V^>  V\.  In  terms  of  the  total  number  of 
ions,  N,  the  two  intensities  will  thus  be  approximately  proportional  to 
o/  Vi  •  X-  and  a/  F2  •  AT2.  So  far  as  the  slant  of  the  decay  curve  is  concerned 
this  is  equivalent  to  a  decrease  in  the  coefficient  of  recombination  and  will 
result  in  making  the  curve  concave  toward  the  axis  of  /.  Absorption  of 
the  emitted  light  will  complicate  the  phenomena  but  will  not  modify  the 
general  result.  The  effect  of  absorption  is  therefore  to  produce  a  change 
in  the  form  of  the  decay  curve  which  is  at  least  in  the  right  direction  to 
account  for  the  observed  deviation  from  linearity. 

In  discussing  the  effect  of  absorption  in  detail  we  shall  assume  that  both 
the  exciting  light  and  the  emitted  light  suffer  absorption,  the  two  coefficients 
of  absorption  being  0  and  /'  respectively.  At  any  depth  a;  below  the  sur- 
face the  intensity  of  the  exciting  light  will  be 


2O6  STUDIES   IN   LUMINESCENCE. 

Assuming  that  ions  are  produced  at  a  rate  proportional  to  E  and  that 
excitation  has  proceeded  for  a  sufficient  time  to  produce  a  steady  condi- 
tion, the  number  of  ions  per  cubic  centimeter  at  any  depth  x  will  be  deter- 
mined by  the  equation 


dt  a 

If  n  is  the  number  of  ions  per  cubic  centimeter  at  the  time  /  we  have 


and  the  light  emitted  per  unit  volume,  denoted  by  i,  is 

pa 


p  being  the  light  emitted  as  the  result  of  one  recombination. 

Since  the  emitted  light  suffers  absorption,  the  amount  contributed  to  the 
total  observed  intensity  by  a  layer  of  thickness  dx  will  be 

.,    _vr        pae~yxdx 
idxe  y  =  -—— 


and  the  total  intensity  is 

/•«      e~yxdx  00     e~yxe~0xdx 

I=paJ0 


where 

a= 

Putting 
so  that 


2y 

J.LJI ,  , 

-  e     2  dx  and 
2 


j_ 


where  m  —  if/ft. 

Successive  partial  integration  gives 

2pa     I"    i       zw+2  2 

~          m2  (a+s)2+ 


,  2-3 


Upon  putting  in  the  limits  this  becomes 

/=  _^?L_   T  FI+  2  .  a-+^^'3  ^r  a/  v  i  ...i 

//  "J 


PHENOMENA   OF,  PHOSPHORESCENCE. 


207 


The  series  in  the  brackets  has  the  value  i  f or  /  =  o  and  as  /  becomes  large 
approaches  the  series 

2  2-3 


If  the  decay  curve  is  plotted  in  the  usual  way  with  7~*  and  /  as  coor- 
dinates it  is  clear  that  the  change  in  slant  in  passing  from  small  values  of  / 
to  large  values  will  depend  upon  5  and  will  be  greatest  when  S  is  greatest. 
The  maximum  deviation  from  linearity  in  the  form  of  the  decay  curve 
which  can  result  from  absorption  will  therefore  be  produced  when  m  =  o, 
i.  e.,  when  r  =  o.  To  determine  whether  absorption  is  sufficient  to  account 
for  the  observed  type  of  curve  we  shall  therefore  consider  this  special  case 
for  which  the  effect  is  greatest. 

For  m  =  o  we  have 

2pa    r°     zdz  2/>a 

Putting  in  the  limits  this  becomes 


J= 


at 


Writing  f)  for  at/a 


r 


The  curve  for  0  and  7  *,  computed  in  accordance  with  this  equation,  is 
plotted  in  Fig.  188,  the  quantity  2/>a/a2/3  being  put  equal  to  unity.  This 
curve  may  be  looked  upon  as  show- 
ing the  relation  between  7-i  and  t 
for  a/a  =  i .  The  curve  for  any  dif- 
ferent value  of  a/a  may  then  be 
found  by  changing  the  vertical  and 
horizontal  scales  to  correspond  to 
the  change  in  a/a. 

The  curve  of  Fig.  188  is  similar 
in  form  to  the  decay  curves  observed 
in  the  case  of  short  excitation. 
Pierce1  has  also  observed  curves  of 
nearly  this  form  with  zinc  sulphide. 
It  is  possible,  therefore,  that  the 
relatively  slight  deviation  from  lin- 
earity in  such  cases  is  to  be  ascribed 


Fig.  188. 


a     e 


to  absorption.  But  in  the  case  of  the  majority  of  decay  curves  it  is  clear 
that  absorption  is  not  to  be  regarded  as  an  important  factor;  for  no 
change  of  scale  can  bring  about  any  close  resemblance  between  the  curve 
of  Fig.  1 88  and  the  decay  curves  usually  observed  for  long  and  moderately 
long  excitation. 

1C.  A.  Pierce,  Physical  Review,  xxvi,  p.  314  (see  Chapter  VI.  Fig.  77).  In  our  own  work  with  this  same 
substance,  "Emanations-pulver,"  we  found  a  decay  curve  of  the  usual  type,  i.  e.,  with  a  well-marked  "shoul- 
der" at  t  =  2o  sec.  (Physical  Review,  xxvin,  p.  50,  or  Chapter  IV,  Fig.  59).  It  is  interesting  to  inquire 
whether  the  difference  is  due  to  the  repeated  heating  and  cooling  to  which  Pierce's  material  was  subjected. 


208  STUDIES   IN   LUMINESCENCE. 


INFLUENCE  OF  IRREGULARITIES  IN  DISTRIBUTION  OF  THE  ACTIVE  MATERIAL- 

Luminescent  substances  are  in  most  cases  solid  solutions.  In  fact  it  is 
doubtful  whether  luminescence  can  occur  in  an  absolutely  pure  substance. 
Pure  calcium  sulphide  for  example  is  not  phosphorescent ;  but  the  addition 
of  a  small  amount  of  some  other  metal,  such  as  manganese  or  copper,  gives 
it  the  power  to  phosphoresce  brilliantly.  The  method  of  preparing  the 
phosphorescent  sulphides  is  such  as  to  bring  about  a  very  intimate  mixture 
of  the  constituents,  and  it  is  natural  to  think  of  the  manganese,  copper,  or 
other  active  material  as  being  dissolved  in  the  sulphide. 

So  little  is  known  regarding  the  nature  of  solid  solutions  that  we  can  not 
say  with  certainty  whether  such  substances  are  to  be  regarded  as  strictly 
homogeneous  or  not.  Especially  in  the  case  of  crystals  it  seems  probable 
that  the  molecules  of  the  solute  may  not  be  uniformly  distributed  through- 
out the  mass  of  the  solvent,  but  may  to  a  greater  or  less  extent  collect  in 
groups  or  minute  crystals.  Any  lack  of  uniformity  in  the  distribution  of 
the  active  substance  will  cause  a  corresponding  variation  in  the  concentra- 
tion of  the  ions  produced  by  the  exciting  light;  and  if  the  nature  of  the 
solvent  is  such  as  to  permit  of  diffusion  the  phenomena  will  be  complicated 
by  the  fact  that  a  redistribution  of  the  ions  will  occur  during  excitation  and 
decay.  Even  without  diffusion,  however,  the  form  of  the  decay  curve  will 
be  modified.  For,  since  the  rate  of  recombination  is  proportional  to  the 
square  of  the  ionic  concentration  the  intensity  of  phosphorescence  will 
decay  at  different  rates  in  different  parts  of  the  mass.  The  effect  on  the 
decay  curve  will  be  similar  to  that  produced  by  absorption ;  in  fact,  the  decay 
curve  is  modified  by  absorption  of  the  exciting  light  only  because  of  the 
resulting  lack  of  uniformity  in  the 'ionic  concentration  at  different  depths 
below  the  surface. 

Our  complete  ignorance  of  the  distribution  of  the  active  material  makes 
it  almost  useless  to  attempt  any  exact  treatment  of  the  problem.  It  is 
possible,  however,  without  questionable  assumptions  or  great  analytical 
complexity,  to  predict  the  general  character  of  the  effect  to  be  expected. 

Let  n  be  the  number  of  ions  per  unit  mass  at  any  point;  n  is  then  some 
unknown  function  of  the  position  of  the  point  in  question  and  of  the  time 
that  has  elapsed  since  the  decay  began.  The  number  of  recombinations 
per  second  will  be  an-  per  unit  volume,  and  the  number  of  recombinations 
for  the  whole  mass  will  bejan^dr,  where  dr  is  an  element  of  volume.  Dis- 
regarding the  absorption  of  the  emitted  light  we  have 


•-*«/ 


Since  ions  are  destroyed  only  by  recombination 

dN 


=  —a  I  n-dr 
dt  J 


where  N  is  the  total  number  of  ions. 

If  «  is  the  volume  average  of  n,  and  n2  the  volume  average  of  n2,  so  that 

dN 


-      i   C    *       N      —      i    /•    2 ,        dN 
«=  -   I   ndr  = — ,       «—  —   I   n-dr,       —  = — 
T."  T  rJ  dt 


am- 


PHENOMENA    OIV PHOSPHORESCENCE.  2OQ 

If  we  put 

P=l^»>      »2  =  P(»)2  =  P-       and     -T7  =  -arp^=-'-A72 
(w)2  r2  at  r2  T 


TV  JV0 

Writing   pi   for  the  average  value  of  p  between  o  and  / 


W--  -J /._»**_*? 

I         ,     dpi  .  <//  T 


,A70 

i    /*'    T.       d  ,     ,\ 
Pi: 


i    /*'  d 

=  -    I    P^/,      i: 

/  -A)  «J 


Although  we  are  unable  to  determine  p  and  pi  as  functions  of  /  it  is  clear 
that  both  will  decrease  as  t  increases.  It  is  clear  also  that  the  change  will 
become  less  rapid  as  the  decay  proceeds,  and  that  both  p  and  pi  will  sooner 
or  latter  become  nearly  constant.  When  7~*  is  plotted  against  t  we  shall 
therefore  obtain  a  curve  which  is  concave  downward  and  which  finally 
becomes  a  straight  line.  In  other  words,  the  decay  curve  will  agree  in 
form  with  the  curves  determined  by  experiment. 

The  effect  upon  the  decay  curve  of  an  irregular  distribution  of  the  active 
substance  may  be  illustrated  by  the  following  simple  case :  Let  the  active 
material  be  uniformly  distributed,  except  that  small  regions  occasionally 
occur  where  the  concentration  is  abnormally  large.  The  whole  volume 
may  thus  be  divided  into  two  parts,  Vi  and  Vz.  The  distribution  of  the 
active  material  is  uniform  throughout  each  part,  but  the  concentration  in 
Vi  is  different  from  that  in  v^.  When  the  substance  is  excited  to  phosphores- 
cence the  ionic  concentration  will  also  differ  in  the  two  regions.  Let  the 
number  of  ions  per  cubic  centimeter  at  the  end  of  excitation  be  n\  in  the 
volume  V!  and  w2  in  the  volume  i'2.  Throughout  the  region  v\  the  intensity 
of  the  light  emitted  per  cubic  centimeter  will  be 

ak 


and  the  total  intensity  at  any  instant  due  to  this  part  of  the  whole  mass 
will  be 


The  light  emitted  by  the  volume  v2  is  given  by  the  similar  expression 


210  STUDIES   IN    LUMINESCENCE. 

and  the  total  intensity  is 


&iO~2  +  (<i«+ft--OV 


where 

i  i  a  a 

It  thus  appears  that  the  form  of  the  decay  curve  is  the  same  as  though 
the  substance  possessed  two  bands,  coincident  as  regards  wave-length,  but 
differing  in  rate  of  decay. 

It  is  not  at  all  unlikely  that  a  condition  approaching  that  assumed  in 
this  illustrative  case  actually  exists  in  most  phosphorescent  substances. 
Upon  considering  the  method  used  in  preparing  the  phosphorescent  sul- 
phides, for  example,  it  seems  probable  that  the  distribution  of  the  active 
substance  will  be  far  from  uniform.  When  the  mixture  is  first  prepared, 
and  before  calcination,  the  active  material  is  unquestionably  in  the  form 
of  small  discrete  masses  distributed  irregularly  through  the  mixture.  Upon 
heating  to  redness  diffusion  will  occur  to  a  greater  or  less  extent,  depending 
upon  the  temperature  and  the  duration  of  heating.  But  even  at  high  temper- 
atures this  will  be  a  slow  process,  and  considerable  variations  in  concen- 
tration are  likely  to  remain  even  after  prolonged  heating.  It  is  to  be 
expected,  therefore,  that  the  phosphorescent  sulphides  will  contain  numer- 
ous nuclei  of  high  concentration  surrounded  in  each  case  by  a  region  where 
the  concentration  is  relatively  small.1 

After  calcination  it  is  often  noticed  also  that  the  phosphorescence  is  very 
far  from  being  uniform  throughout  the  mass.  Owing  probably  to  accidental 
differences  in  concentration,  or  to  differences  in  the  heat  treatment,  the 
phosphorescence  often  differs  greatly  in  intensity  and  even  in  color  in 
different  parts  of  the  same  mass.  A  phosphorescent  powder  made  from 
such  a  mass,  while  presenting  the  appearance  of  homogeneity  to  the  un- 
aided eye,  would  differ  greatly  from  point  to  point  in  the  concentration  of 
the  active  material.2  Even  if  the  mixture  were  so  perfect  that  no  irregu- 
larities could  be  detected  with  the  microscope,  a  wide  deviation  from  line- 
arity in  the  decay  curve  is  to  be  expected.  It  is  a  significant  fact  that 
with  one  exception  all  decay  curves  thus  far  recorded  have  been  determined 
with  powders  prepared  in  practically  the  same  way  that  the  phosphorescent 
sulphides  are  prepared.  In  fact,  most  of  the  substances  tested  were  sul- 
phides. The  exception  noted  above  was  natural  willemite,3  in  which  case 
the  variation  in  brightness  over  the  surface  tested  was  plainly  visible. 

'Since  a  more  complete  diffusion  of  the  active  material  will  result  from  prolonged  heating  it  is  to  be 
expected,  other  conditions  being  the  same,  that  the  duration  of  phosphorescence  will  be  prolonged  by  in- 
creasing the  time  of  heating.  This  agrees  with  the  facts  observed  in  the  preparation  of  the  phosphorescent 
sulphides. 

2While  we  are  chiefly  concerned  at  present  with  the  influence  of  lack  of  homogeneity  upon  the  decay  curve, 
it  can  scarcely  be  doubted  that  the  effect  upon  the  phosphorescence  spectrum  is  fully  as  important.  For 
some  reason  different  parts  of  the  mass  are  differently  affected  by  the  process  of  calcination.  This  may  be 
due  to  differences  in  concentration  throughout  the  mass,  or  to  the  fact  that  the  effect  of  the  surrounding 
gas  varies  from  point  to  point.  When  such  a  mass  is  powdered  and  mixed  the  effect  is  the  same  as  though  we 
were  to  make  an  intimate  mixture  of  several  entirely  different  phosphorescent  substances.  Each  constituent 
has  its  own  band  or  group  of  bands,  characterized  by  definite  wave-lengths  and  periods  of  decay,  which 
differ  according  to  the  conditions  of  preparation.  It  is  not  surprising  that  the  phosphorescence  spectrum 
of  such  a  material  is  complex;  and  we  can  scarcely  expect  simple  laws  to  apply  to  any  of  the  phenomena 
exhibited  by  such  a  mixture. 

•The  decay  of  phosphorescence  in  a  specimen  of  willemite  possessing  a  long-time  phosphorescence  has 
been  studied  by  Nichols  and  Merritt,  Physical  Review,  xxni,  p.  52,  Fig.  53.  Willemite  whose  phosphor- 
escence dies  out  with  great  rapidity  has  been  studied  by  Waggoner,  Physical  Review,  xxvn,  p.  209.  See 
also  Chapters,  IV  and  VII. 


PHENOMENA   OF  PHOSPHORESCENCE.  211 

It  thus  appears  that  irregularities  in  the  distribution  of  the  active  sub- 
stance are  sufficient  to  explain  the  deviation  from  linearity  in  all  the  decay 
curves  thus  far  observed.  Such  irregularities  of  distribution  are  not  merely 
probable,  but  in  many  cases  are  perfectly  obvious.  But  while  the  dis- 
tribution of  the  active  material  is  probably  in  all  cases  an  important  factor, 
it  can  not  be  the  only  factor  of  importance.  With  Balmain's  paint,  Pierce 
has  found  that  the  decay  curve,  which  possesses  the  usual  shoulder  at 
ordinary  temperatures,  becomes  almost  exactly  linear  at  a  temperature  of 
740,1  while  for  higher  temperatures  it  again  shows  a  curvature  of  the  usual 
kind.  It  appears  highly  improbable  that  such  changes  are  brought  about 
by  temporary  changes  in  the  distribution  of  the  active  substance.  Non- 
uniformity  in  the  distribution  of  the  active  material  also  offers  no  explana- 
tion of  the  phenomena  of  hysteresis  or  of  the  effect  of  the  infra-red  rays. 

DIFFUSION   EFFECTS. 

Whenever  irregularities  exist  in  the  distribution  of  the  active  substance 
there  will  be  a  tendency  for  diffusion  to  occur.  Under  ordinary  conditions 
this  tendency  is  probably  neutralized  by  forces  which  act  to  keep  the  dis- 
tribution unaltered,  and  so  long  as  the  substance  remains  in  the  molecular 
form  the  condition  is  to  be  regarded  as  a  stable  one  for  the  temperature  and 
pressure  at  which  the  phosphorescent  substance  normally  exists. 

But  when  the  material  is  excited  to  luminescence  a  part  of  the  active 
substance  will  be  dissociated,  and  since  the  resulting  ions  will  possess  a 
different  mobility  and  will  be  acted  upon  by  different  forces  from  those 
that  determine  the  behavior  of  the  original  neutral  molecules,  the  condition 
of  equilibrium  will  be  destroyed,  and  some  change  in  the  distribution  of 
the  active  substance  is  to  be  expected. 

An  exact  discussion  of  the  effects  of  diffusion  would  present  great  diffi- 
culties; for  the  fact  that  diffusion  and  recombination  occur  at  the  same 
time  greatly  complicates  the  analytical  treatment.  It  is  probable  also 
that  in  crystals  the  diffusion  constant  will  be  different  for  different  direc- 
tions. It  is  clear,  however,  that  the  influence  of  diffusion  upon  the  form 
of  the  decay  curve  must  be  similar  to  that  produced  by  an  irregular  dis- 
tribution of  ions  without  diffusion.  In  fact,  the  discussion  of  the  preceding 
section  applies  without  modification  to  the  case  of  substances  in  which 
diffusion  of  the  ions  may  occur.  Diffusion,  however,  will  increase  the 
rapidity  with  which  uniformity  of  ionic  concentration  is  approached  dur- 
ing decay ;  and  since  diffusion  will  be  most  active  when  large  concentration 
gradients  exist,  the  effects  of  diffusion  will  be  greatest  in  the  early  stages 
of  decay.  In  cases  where  diffusion  is  an  important  factor  we  should  there- 
fore expect  a  sharper  curvature  in  the  early  part  of  the  curve,  and  a  more 
rapid  approach  to  linearity,  than  in  cases  where  diffusion  is  absent. 

It  is  not  impossible  that  diffusion  is  sometimes  important  even  when 
there  is  nearly  complete  uniformity  in  the  distribution  of  the  active  sub- 
stance. Owing  to  the  absorption  of  the  exciting  light  the  ionization  pro- 
duced in  the  surface  layer  will  be  more  intense  than  that  produced  at  points 
beneath  the  surface.  If  the  absorption  is  large  a  large  gradient  may  thus 

'Physical  Review,  xxvi,  p.  458.    See  also  Chapter  VI. 


212  STUDIES   IN    LUMINESCENCE. 

be  produced  in  the  ionic  concentration,  and  diffusion  from  the  surface 
layers  inward  will  bring  about  a  change  in  the  decay  curve  similar  in  char- 
acter to  that  caused  by  irregularities  in  the  distribution  of  the  active  sub- 
stance. Diffusion  produced  in  this  way  would  ultimately  result  in  an  appre- 
ciable diminution  in  the  surface  concentration  of  the  active  material,  and 
we  should  therefore  expect  that  the  intensity  of  luminescence  would  be 
diminished  in  such  a  substance  by  prolonged  excitation.  No  effect  of  this 
kind  has  been  observed  by  us  in  the  case  of  Sidot  blende,  but  the  obser- 
vations of  Werner1  with  a  SrZn  compound  show  evidence  of  fatigue  resulting 
from  prolonged  excitation. 

The  change  in  the  form  of  the  decay  curve  due  to  changes  in  the  duration 
of  excitation  may  be  explained,  at  least  in  a  general  way,  as  a  result  of 
diffusion.  Diffusion  of  the  ions  will  occur  during  excitation  as  well  as 
during  decay.  After  prolonged  excitation,  therefore,  the  volume  occupied 
by  ions  will  be  greater  than  after  short  excitation,  and  the  rate  of  decay — in 
other  words  the  slant  of  the  decay  curve — will  be  correspondingly  reduced.2 

Diffusion  also  offers  an  explanation  of  the  phenomena  of  hysteresis  in 
phosphorescent  substances.  After  prolonged  excitation  and  subsequent 
decay  the  neutral  molecules  that  result  from  recombination  will  be  dis- 
tributed through  a  larger  volume  than  before.  Since  the  original  distribution 
of  the  active  material  was  a  stable  one  there  will  be  a  gradual  return  to  the 
normal  distribution.  But  this  will  be  a  slow  process  and  may  well  require 
several  days  for  its  completion.  In  the  mean  time  the  material  is  in  such 
a  condition  that  the  decay  following  renewed  excitation  will  be  more 
gradual  than  the  normal,  even  for  a  short  excitation.  The  spreading  out 
of  the  active  substance  by  diffusion,  which  would  normally  require  a  long 
excitation,  has  already  been  accomplished  by  the  preceding  excitation, 
whose  effects  have  not  yet  disappeared.  Hysteresis  effects  such  as  those 
discussed  in  Chapter  IV  are  therefore  to  be  expected. 

If  this  explanation  of  hysteresis  is  correct  the  effect  of  the  infra-red  rays 
must  be  to  facilitate  the  return  of  the  substance  to  its  normal  condition ; 
in  other  words,  toincrease  the  rapidity  of  the  diffusion  by  which  the  original 
distribution  of  the  active  material  is  restored.  It  is  natural  to  expect  such 
an  effect  in  substances  which  are  able  to  absorb  the  infra-red  rays.  But  if 
the  restoration  is  accomplished  by  the  diffusion  of  neutral  ions  the  rapidity 
of  the  action  is  surprising,  for  with  strong  infra-red  rays  we  have  found  that 
an  exposure  of  only  a  few  seconds  is  sufficient  to  restore  Sidot  blende  to 
its  standard  condition.3 

INFLUENCE   OF   IONIC   GROUPING. 

In  discussing  thermo-luminescence  and  the  effect  of  infra-red  rays  upon 
phosphorescence  Wiedemann  and  Schmidt4  have  suggested  that  some  of 
the  ions  produced  during  excitation  form  semi-stable  combinations  or 

'A.  Werner,  Ann.  der  Phys.,  24,  p.  164,  1907. 

2If  the  volume  occupied  by  ions  is  PI  for  short  excitation  and  ra  for  long  excitation,  the  ionic  concentra- 
tions will  be  n/vi  and  n/rs  respectively,  and  the  two  intensities  of  phosphorescence  will  be  proportional,  to 
an2 /"i  and  an/^/vi.  Prolonged  excitation  therefore  produces  an  effect  which  is  equivalent  to  a  diminution 
in  the  coefficient  of  recombination. 

'See  Chapter  V. 

4Ann.  der  Phys.,  56,  p.  247,  1895. 


PHENOMENA   0$   PHOSPHORESCENCE.  213 

groups  with  the  neutral  molecules  of  the  solvent,  and  that  these  groups  may 
afterwards  be  broken  down,  and  the  ions  liberated,  by  rise  in  temperature 
or  by  the  absorption  of  infra-red  rays.  The  formation  of  such  groups  as 
the  result  of  ionization  seems  extremely  probable,  especially  in  the  case  of 
solids,  and  can  scarcely  fail  to  be  of  importance  in  any  satisfactory  theory 
of  phosphorescence.  Introducing  such  additional  hypotheses  as  are  neces- 
sary to  give  definiteness  to  the  suggestion  of  Wiedemann  and  Schmidt,  let 
us  consider  what  the  influence  of  such  ionic  groups  will  probably  be. 

We  shall  assume  that  the  first  effect  of  the  exciting  light  is  to  produce 
dissociation  in  a  part  of  the  active  material.  The  dissociation  assumed 
may  be  either  chemical  or  electrolytic1  and  if  of  the  latter  type  it  may  either 
be  similar  to  the  dissociation  of  ordinary  electrolysis,  or  may  consist  of  the 
expulsion  from  the  molecule  of  one  or  more  electrons,  and  thus  resemble 
more  closely  the  ionization  of  a  gas  by  X-rays.  For  the  sake  of  definiteness 
we  shall  assume  that  the  effect  of  the  exciting  light  is  to  produce  such  violent 
vibrations  as  to  liberate  a  single  electron  from  the  molecule. 

The  two  ions  produced  in  this  type  of  dissociation  will  differ  greatly  in 
mobility.  The  negative  ion,  owing  to  its  small  mass,  will  possess  a  velocity 
hundreds  of  times  greater  than  that  of  the  heavy  positive  ion,  and  in  con- 
sequence will  move  about  in  the  substance  with  considerable  freedom. 
While  the  electrons  will  at  times  attach  themselves  to  the  molecules  of  the 
solid  solvent  this  condition  will  usually  be  only  temporary.  We  may 
assume  in  general  that  a  constant  fraction  of  the  whole  number  of  negative 
ions  consists  of  electrons  that  are  moving  freely.  The  positive  ions  on  the 
other  hand  will  possess  only  a  small  mobility.  While  some  of  these  ions 
will  remain  free,  it  is  to  be  expected  that  many  will  attach  themselves  to 
molecules  of  the  solvent  or  to  undissociated  molecules  of  the  active  sub- 
stance. It  is  to  be  noted  that  the  small  velocity  of  the  positive  ions  makes 
it  probable  that  the  groups  formed  by  the  union  of  a  positive  ion  with  a 
neutral  molecule  will  be  more  permanent  than  similar  groups  formed  by  the 
negative  ion,' 

The  collisions  between  positive  and  negative  ions,  which  lead  to  the  more 
or  less  gradual  decay  of  the  ionized  condition  after  the  exciting  light  has 
ceased  to  act,  will  be  of  three  different  kinds:  (i)  collisions  between  a  nega- 
tive ion  and  a  free  positive  ion;  (2)  collisions  between  a  negative  ion  and  a 
positive  ion  that  has  attached  itself  to  a  neutral  molecule  of  the  solvent,  and 
(3)  collisions  between  a  negative  ion  and  a  positive  ion  that  is  attached  to  a 
neutral  molecule  of  the  active  substance.  The  number  of  modes  of  recom- 
bination may  in  fact  be  greater  than  three,  since  the  positive  ion  may  become 
the  nucleus  of  more  complicated  molecular  groups.  But  we  shall  restrict 
the  discussion  to  cases  in  which  there  are  three  modes  of  recombination. 

When  recombination  occurs  it  is  to  be  expected  that  vibrations  will  be 
set  up  in  the  resulting  neutral  molecule,  and  these  vibrations,  in  the  theory 
here  considered,  are  assumed  to  be  the  source  of  the  light  emitted  during 
the  phosphorescence.  But  the  vibrations  corresponding  to  the  different 

"Since  the  electro-magnetic  disturbance  that  constitutes  light  can  get  a  hold  on  the  molecules  of  the  active 
material  only  by  exerting  forces  upon  the  electrical  charges  in  the  molecule,  and  will  always  tend  to  separate 
the  positive  and  negative  parts,  it  appears  probable  that  the  first  effect  of  the  exciting  light  is  always  to 
produce  some  type  of  electrolytic  dissociation,  and  that  any  chemical  changes  which  may  be  exhibited  are 
secondary  effects. 


214  STUDIES   IN   LUMINESCENCE. 

modes  of  recombination  will  probably  differ  in  violence,  in  frequency,  and 
in  radiating  power. 

Of  the  total  number  n  of  positive  ions  at  any  time  t  let  <pn  be  free,  and 
\l/n  attached  to  neutral  molecules  of  the  active  substance;  then  (i  —  <p  —  \{/)n 
will  be  attached  to  molecules  of  the  solvent.  <p  and  \f/  are  proper  fractions 
whose  sum  is  less  than  unity,  and  which,  in  general,  are  functions  of  t; 
for  the  distribution  of  the  positive  ions  will  be  subject  to  alteration  during 
the  decay  of  phosphorescence  as  well  as  during  excitation.  The  number 
of  recombinations  between  a  free  positive  ion  and  an  electron  in  unit  time 
will  be  proportional  to  the  number  of  free  positive  ions  <pn,  and  to  the 
number  of  free  negative  ions  n,  and  may  therefore  be  put  equal  to  anpn~, 
where  tti  is  the  coefficient  of  recombination.  If  the  energy  radiated  as 
light  due  to  one  recombination  is  pi  the  intensity  of  the  phosphorescence 
due  to  this  mode  of  recombination  will  be 

Similarly 

/2 

for  the  other  two  types  of  recombination. 

The  total  phosphorescent  light  will  thus  consist  of  three  parts.  If  the 
different  modes  of  recombination  give  rise  to  vibrations  of  different  fre- 
quency the  phosphorescence  spectrum  will  consist  of  three  bands  which 
decay  at  different  rates,  and  which  may  differ  widely  in  intensity. 

The  determination  of  the  law  of  decay  of  phosphorescence  upon  the  basis 
of  the  theory  just  outlined  is  thus  seen  to  involve  the  determination  of 
n,  <p,  and  ^  as  functions  of  /.  In  the  general  case  the  solution  presents 
difficulties  that  are  wellnigh  insurmountable.  The  problem  may  be  sim- 
plified, however,  by  an  assumption  which,  while  doubtless  not  exactly 
true,  probably  gives  in  the  majority  of  cases  a  close  approximation  to  the 
actual  conditions.  It  is  assumed,  namely,  that  ai=a2  =  a3;  in  other  words, 
the  probability  that  a  collision  will  result  in  recombination  is  the  same  for 
the  three  types  of  collision.  In  this  case 

/  %  dn 

—-—  —a<pn2—a4<n-—a(i—<p  —  \l/)rii=  —anz 

(2) 


where  «o  is  the  number  of  positive  (or  negative)  ions  when  excitation  ceases. 
To  determine  <p  and  ^  as  functions  of  /  it  is  necessary  to  make  some 
assumption  regarding  the  rate  at  which  the  free  positive  ions  attach  them- 
selves to  neutral  molecules.  Since  the  number  of  neutral  molecules  is 
presumably  large  as  compared  with  the  number  of  ions,  it  is  reasonable 
to  assume  that  the  rate  at  which  new  groups  are  formed  is  proportional 
to  the  number  of  ions  that  are  still  free  to  enter  into  such  combination. 
In  other  words,  we  may  assume  k\tpn  and  k^n  as  the  rates  of  formation  of 
new  groups  of  the  two  possible  types.  Since  the  positive  ions,  whether 
free  or  attached,  are  also  recombining  with  the  negative  ions  we  have 

(3)  3  (<pn)  = 

at 


PHENOMENA   OIJ  PHOSPHORESCENCE.  215 

While  positive  ions  will  occasionally  break  loose  from  the  groups  to 
which  they  have  attached  themselves  and  return  to  the  "free"  condition, 
this  will  occur  only  rarely  on  account  of  the  sluggishness  with  which  these 
ions  move.  The  small  positive  term  due  to  this  cause  which  should  appear 
in  the  right  hand  member  of  equation  (3)  is  therefore  negligible  and  may  be 
omitted.  At  high  temperatures,  however,  and  perhaps  under  other  special 
conditions,  the  omission  of  this  term  will  no  longer  be  permissible.  If  the 
molecular  movements  are  sufficiently  violent,  due  to  high  temperature  or 
other  causes,  the  formation  of  groups  may  even  be  prevented  altogether. 
Such  cases  will  be  considered  later. 

Equation  (3)  may  be  written 

d  dn        dp 

-j.(<pn)  =  <p  j.+n—  = 
at  at         at 

Remembering  that  dn/dt=  —an2  this  becomes 


where  m  =  ki-\-kz  and  <£>o  is  the  value  of  <p  for  t  =  o. 

In  the  case  of  the  positive  ions  that  are  attached  to  neutral  molecules  of 
the  active  substance  recombinations  are  occurring  with  negative  ions  at 
the  rate  a\(/ir,  while  new  groups  are  being  formed  by  the  attachment  of 
free  positive  ions  at  the  rate  ki<pn 

d 

'  dt 
or 

dn         d\f/ 
(5)  ^      ~\~n       =  — o.\l/), 

Since 

dn  =  _   .2 

we  have 

*_i   -k      .--.  -    ,          — __m<+cor 


. 
dt  m 

Putting  \[/0  for  the  value  of  ^  when  /  =  o 

(6)  lA  =  v 


m 


For  the  intensities  of  the  three  constituents  of  the  phosphorescent  light 
we  have  therefore 


216  STUDIES   IN   LUMINESCENCE. 

Since  we  have  usually  plotted  the  reciprocal  square  root  of  the  intensity 
of  phosphorescence,  it  will  be  convenient  to  use  the  same  procedure  here. 

i/V7>  -=^=  *im 


Replacing  certain  groups  of  constant  terms  by  single  letters  for  brevity, 
these  expressions  may  be  written 


T/  IT        i/rco+a* 

I/V/3  =       /    . 


If  a  curve  is  plotted  for  i/V  /i  in  accordance  with  the  above  equation  it 
Is  found  to  be  of  the  type  shown  in  Fig.  190  for  7  =  0.  The  curve  for  i/ V  It 
and  i/V  /2  is  concave  upward  during  the  early  stages  of  decay,  becoming 
practically  straight  when  t  is  so  large  as  to  make  e~mt  inappreciable. 

No  decay  curves  corresponding  to  either  of  these  types  has  been  observed. 
It  must  be  remembered,  however,  that  the  effects  of  absorption,  diffusion, 
and  irregular  distribution  have  been  neglected  in  the  present  discussion. 
It  is  probable  that  one  or  more  of  these  sources  of  disturbance  are  present, 
in  which  case  the  theoretical  decay  curves  would  be  modified  in  such  a  way 
as  to  make  them  correspond  more  nearly  with  the  curves  actually  observed. 
If  m  is  large,  for  example,  the  curve  for  72~*  will  be  straight  except  in  the 
immediate  neighborhood  of  /  =  o.  Irregularities  in  the  distribution  of  the 
active  substance  would  then  produce  a  deviation  from  linearity,  as  previ- 
ously discussed.  It  appears  probable  that  the  observed  facts,  in  the  cases 
of  phosphorescence  thus  far  studied,  are  best  explained  in  this  way.  It  is 
interesting  to  note,  however,  that  curves  may  be  obtained  as  the  result  of 
ionic  grouping  which  agree  very  closely  with  the  experimental  curves,  even 
on  the  assumption  of  a  uniform  distribution  of  the  active  material.  If,  for 
example,  /i  and  72  both  contribute  to  the  phosphorescence,  while  h  is  zero 
or  negligible 


(7)  i/V7=-^ 

where 

,4  =  />' 


i.o<i. 
m  /  m 


PHENOMENA    OF,  PHOSPHORESCENCE. 


In  case  all  three  modes  of  recombination  contribute  to  the  phosphores- 
cence we  obtain  a  similar  expression  for  7~*,  except  that  A  and  B  now  have 
the  values 

A  = 


m 


m 


B  = 


pad— 
m  m 


400 


300 


200 


100 


20 


4-0 


60 


80  100 

Seconds 
Fig.  189. 


120 


I4-O 


160 


Observed  decay   curves  for  different  durations  of  excitation  compared  with  curves  computed 

from  the  equation 

i  i/nt+a/ 


The  times  of  excitation  and  the  relative  numerical  values  of  the  constants  used  in  computation  are  as  follows : 
Curve.       Excitation.  y  i/No  <JB 

E  do     sec.  0.36  72          i        } 

D  37  0.4  167          z.iSl   0=4.8 

C  ii. 6  o.i  336          4        Im=o.i6 

B  54  o.i  650         4-35J 

In  Fig.  189  several  curves  have  been  plotted  for  7~*  by  means  of  this 
equation.  Values  of  the  constants  have  been  determined  by  trial  so  as 
to  make  these  curves  correspond  as  nearly  as  possible  with  the  series  of 
experimental  curves  obtained  with  Sidot  blende  by  varying  the  time  of 
excitation.1  Observational  points  are  indicated  by  circles.  It  will  be 
seen  that,  except  in  the  case  of  one  point  on  curve  D,  the  agreement  is  highly 
satisfactory. 

To  give  an  idea  of  the  character  of  the  decay  curves  which  might  result 
from  the  presence  of  grouped  ions  (disturbances  due  to  diffusion,  absorp- 
tion, etc.,  being  neglected)  we  may  write  equation  (7)  in  the  form 


'Nichols  and  Merritt,  Physical  Review,  xxm.  p.  45.     See  also  Chapter  IV. 


218 


STUDIES   IN   LUMINESCENCE. 


where 


T  = 


and    y  = 


Q.\ 


O.Z 


The  first  factor  alone  plots  as  a  straight  line.  The  deviation  from 
linearity  in  the  curve  for  7~~*  is  therefore  determined  by  the  second  factor 
T.  Putting  m  =  I  we  have  computed  T  as 
a  function  of  /  for  several  values  of  7,  the 
results  being  plotted  in  Fig.  190.  For 
values  of  7  ranging  from  o.i  to  0.5  the 
curves  are  of  such  a  character  as  to  cor- 
respond with  the  experimental  curves  for 
J~  •  But  for  smaller  values  of  7  a  double 
curvature  is  shown  which  is  not  found  in 
any  of  the  curves  that  we  have  deter- 
mined. There  is,  however,  an  indication 
of  such  a  double  curvature  in  some  of  the 
results  of  Pierce.  It  is  clear  that  disturb- 
ances, due,  for  example,  to  irregularities 
in  the  distribution  of  the  active  material, 
might  so  alter  the  early  part  of  the  curve 
as  to  eliminate  any  peculiarities  of  this 
nature.  In  fact,  there  is  at  present  so 
much  uncertainty  regarding  the  relative 
importance  of  the  different  factors  that 
influence  the  form  of  the  curve  for  small 
values  of  /  that  it  is  difficult  to  reach  any 
definite  conclusions  concerning  this  part 
of  the  curve.  Experiments  bearing  upon 


60 


Fig.  190. 

Curves  showing  relation  between  T  and  t  for 
different  values  of  y  where 

' 


r- 


. 

\ 


the  distribution  of  the  active  material,  the  rate  of  diffusion  of  the  ions,  and 
related  matters  are  greatly  needed. 

HYSTERESIS,  TEMPERATURE  EFFECTS,  ETC.,  EXPLAINED  BY  IONIC  GROUPING. 

To  account  for  the  hysteresis  exhibited  by  phosphorescent  substances, 
in  other  words,  the  effect  of  a  previous  exposure  upon  the  phosphorescence 
produced  by  a  given  excitation,  it  is  necessary  to  consider  the  essential 
difference  that  probably  exists  between  the  groups  formed  by  the  union  of 
positive  ions  with  neutral  molecules  of  the  active  substance  and  those 
formed  by  the  attachment  of  positive  ions  with  molecules  of  the  solvent. 
For  the  sake  of  brevity,  as  well  as  for  the  reasons  that  will  appear  shortly, 
we  shall  refer  to  the  former  as  favorable  groups  and  to  the  latter  as  unfavor- 
able groups. 

We  have  already  introduced  the  assumption  that  light  is  produced  by 
the  recombination  of  a  negative  ion  with  a  favorable  group,  while  the  re- 
combination of  the  unfavorable  groups,  at  least  in  some  cases,  gives  out  no 
light.  A  difference  is  to  be  expected  also  in  the  behavior  of  the  neutral 
molecules  that  result  from  recombination  in  the  two  cases.  A  favorable 
group  consists  in  a  positive  ion  attached  to  at  least  one  neutral  molecule 
of  the  same  sort.  It  seems  natural  to  expect  that  forces  similar  to  those 
that  hold  together  the  molecules  of  a  crystal  may  cause  this  grouping  to 


PHENOMENA   O^f  PHOSPHORESCENCE.  219 

persist  even  after  recombination  has  occurred.  In  the  case  of  the  unfavor- 
able groups  this  tendency  to  persist  can  scarcely  be  present  in  the  same 
degree  if  at  all. 

The  condition  of  the  phosphorescent  substance  is  thus  different  after 
phosphorescence  has  ceased  from  what  it  was  before  excitation.  The 
difference  consists  in  the  presence  in  the  mass  of  a  larger  number  of  grouped 
molecules  of  the  active  substance,  which  are  so  intimately  connected  that 
when  one  member  of  the  group  is  dissociated  during  subsequent  excitation 
its  positive  ion  is  in  a  position  to  form  immediately  one  of  the  groups 
favorable  to  phosphorescence.  After  the  substance  has  been  excited  it  is 
therefore  in  a  condition  which  enables  a  subsequent  excitation  to  produce 
a  larger  proportion  of  favorable  groups  than  would  be  produced  by  a  simi- 
lar excitation  of  the  fresh  substance.  In  other  words,  the  ^0  of  equation 
(7)  is  increased. 

During  excitation  we  have  dissociation  and  recombination  taking  place 
at  the  same  time;  and  the  recombinations  that  occur  during  excitation 
will  bring  about  the  same  change  in  the  condition  of  the  substance  that  we 
have  assumed  during  decay.  The  value  of  \f/o  will  therefore  increase  with 
the  duration  of  exposure  to  the  exciting  rays.  Prolonged  excitation,  up 
to  the  point  where  saturation  is  reached,  also  increases  the  number  of  ions, 
i.  e.,  the  value  of  wo.  A  series  of  decay  curves  for  different  times  of  exposure 
should  therefore  resemble  the  curves  of  Fig.  189,  which  are  computed  from 
equation  (7)  by  giving  progressively  increasing  values  to  the  constants 
to  and  «o- 

The  new  condition  in  which  a  substance  is  left  after  phosphorescence 
can  scarcely  be  one  of  complete  stability.  The  natural  and  stable  arrange- 
ment of  the  molecules  is  that  of  the  substance  before  it  has  been  disturbed 
by  the  action  of  light.  It  is  to  be  expected,  therefore,  that  there  will  be  a 
more  or  less  gradual  return  to  the  normal  state  after  the  light  has  ceased 
to  act.  The  recombination  of  the  ions  produced  by  excitation,  with  the 
accompanying  phosphorescence,  forms  only  one  stage  in  the  complete 
return  to  the  normal  state,  and  is  followed  by  a  more  gradual  breaking 
down  of  the  molecular  groups  resulting  from  recombination.  We  thus 
have  an  explanation  of  the  effect  of  rest.  The  effect  of  exposure  to  infra- 
red rays  and  of  elevation  of  temperature  is  to  hasten  the  return  to  the 
normal  state  by  increasing  the  rate  at  which  these  groups  disintegrate. 

To  account  for  the  action  of  the  infra-red  rays  during  excitation  and 
decay  it  is  only  necessary  to  assume  that  these  rays  also  have  the  power 
of  breaking  down  the  "favorable  groups."  In  the  case  of  Sidot  blende 
the  effect  on  the  unfavorable  groups  appears  to  be  inappreciable.  Upon 
exposure  to  infra-red  during  decay  the  first  result  is  to  diminish  the  number 
of  favorable  groups,  and  to  correspondingly  increase  the  number  of  free 
positive  ions.  This,  by  itself,  will  not  greatly  alter  the  intensity  of  phos- 
phorescence; for  in  the  phosphorescence  of  Sidot  blende  the  recombina- 
tion of  a  free  positive  ion  appears  to  be  nearly  or  quite  as  effective  as  the 
recombination  of  a  favorable  group.  But  an  increase  in  the  number  of 
free  ions  causes  an  increase  in  the  rate  at  which  unfavorable  groups  are 
formed.  The  positive  ions  that  are  shaken  loose  from  the  favorable  groups 
therefore  pass  quickly  into  the  inactive  condition,  and  a  rapid  diminution 


220  STUDIES   IN   LUMINESCENCE. 

in  the  intensity  of  phosphorescence  results.  The  rate  of  decrease  of 
course  depends  upon  the  intensity  of  the  active  rays. 

If  the  infra-red  rays  are  allowed  to  act  for  a  short  time  and  are  then 
cut  off,  the  condition  of  the  phosphorescent  substance  will  differ  in  two 
respects  from  that  which  it  would  have  reached  during  ordinary  decay: 
(i)  the  number  of  favorable  groups  is  less  than  it  would  have  been  with- 
out the  action  of  the  longer  waves;  (2)  the  number  of  free  ions  is,  at  least 
to  some  extent,  in  excess  of  the  normal.  After  the  infra-red  rays  have 
ceased  to  act,  however,  the  free  ions  will  soon  form  groups  again,  either 
favorable  or  unfavorable,  and  the  decay  curve  will  quickly  return  to  the 
standard  form.  But  the  number  of  favorable  groups  will  be  less  than  if 
the  infra-red  rays  had  not  acted.  The  effect  of  exposure  to  the  longer 
waves  is  simply  to  bring  the  substance  quickly  into  the  same  condition  that 
it  would  ordinarily  acquire  only  after  a  much  longer  period  of  decay.  The 
theory  here  discussed  is  thus  seen  to  be  in  agreement  with  the  experimental 
results  recorded  in  Chapter  V. 

It  seems  probable  that  in  some  substances  certain  rays,  presumably 
in  the  infra-red,  may  have  the  effect  of  breaking  down  the  unfavorable 
as  well  as  the  favorable  groups.  In  such  cases  exposure  to  these  rays 
would  probably  bring  about  an  increase  in  the  brilliancy  of  phosphores- 
cence instead  of  a  decrease.  This  is  the  case  with  certain  of  the  phospho- 
rescent sulphides.  The  same  effect  would  be  produced  in  substances  where 
the  recombination  of  a  free  positive  ion  gives  out  more  light  than  that  of  an 
attached  ion,  i.  e.,  where  p\>  pz- 

In  cases  like  that  of  Sidot  blende  the  form  of  the  decay  curve  as  modified 
by  exposure  to  infra-red  rays  may  be  determined  as  follows  : 

We  shall  assume  that  the  rate  at  which  favorable  groups  are  broken 
down  is  proportional  to  the  intensity  of  the  active  rays  I,  and  the  number 
of  favorable  groups  present  (\l/n).  We  therefore  have 

(8)  -r(^«)  =  —  ai/'w2  —  R^n  —  k\<pn 

at 

where  R  is  proportional  to  Ir. 

The  destructive  effect  of  the  infra-red  is  so  great,  even  for  rays  of  small 
intensity,  while  ki  is  so  small,  that  the  last  term  knpn  may  in  general  be 
neglected.  It  is  only  in  the  case  of  exposure  to  very  weak  rays,  or  with 
substances  which  show  the  infra-red  effect  in  small  intensity,  that  the 
omission  of  this  term  will  lead  to  appreciable  errors.  Equation  (8)  there- 
fore becomes 


,dn   ,      d\L  d\L  d\l/ 

-  +  »••;-  =  -  atn-  +  n  ,    =  -  af«2  -  R+n 
at         at  at  an 


(9) 


where  /  is  reckoned  from  the  time  when  exposure  to  the  infra-red  rays  begins. 
If  we  take  /  as  the  time  that  has  elapsed  since  the  end  of  excitation  we  have 


PHENOMENA   OF,  PHOSPHORESCENCE-  221 

where  /i  is  the  time  at  which  the  longer  waves  begin  to  act  and  i/'i  is  the 
value  of  \l/  at  this  time. 

To  determine  the  number  of  free  positive  ions  <pn  after  the  infra-red  rays 
begin  to  act  we  have  the  equation 

-  (<f>n)  =  —a<pn?-\-R\f/n  —  knpn 
dt 

dn       d<p  d<p 

<P~T  -\-n-~  —  —  <pa.nz-\-n-j    =  —  ouptr  +  R\f/n  —  kz<f>n 
dt        dt  dt 


at 

(10)  v- 

KZ  —  -- 

An  interesting  special  case,  corresponding  to  the  experimental  condi- 
tions in  much  of  our  work  wTith  Sidot  blende,  is  that  in  which  the  exposure 
to  infra-red  rays  begins  at  once  when  excitation  ceases,  i.  c.,  /i  =  o.  In  this 
case 


KZ  — 

I  =  pia<pnz+p2a2tnz  =  (A 
where 


—    -  M 
KZ  —  KJ 


For  the  special  case  where 
=  =<n  = 


If  the  infra-red  rays  are  intense  so  that  R/kz  is  large 
i  i/n0-\-at 


and  in  the  extreme  case  where  R  may  be  treated  as  infinite 

i  I/MO  +  <M 

V7  = 


This  gives  a  curve  having  the  same  form  as  the  observational  curve  shown 
in  Fig.  72.  The  data  of  Fig.  70  will  be  found  to  give  a  curve  of  the  same 
form  for  i/V/',  but  the  curve  has  been  plotted  only  in  the  case  of  Fig.  72. 

At  high  temperatures  the  increased  activity  of  the  molecular  movements 
will  tend  to  prevent  the  formation  of  groups,  or  rather  to  destroy  the 
groups  almost  as  quickly  as  they  are  formed.  All  those  phenomena  that 
depend  upon  the  existence  of  groups  will  therefore  become  less  prominent 


222  STUDIES   IN   LUMINESCENCE. 

as  the  temperature  rises.  Sooner  or  later  a  temperature  will  be  reached 
where  groups  can  no  longer  exist.  At  temperatures  above  this  point  we 
should  expect  no  effect  from  exposure  to  the  infra-red,  and  all  hysteresis 
effects  should  disappear.  The  experiments  of  Pierce  show  that  hysteresis 
still  persists  in  Balmain's  paint  at  133°  C.  No  other  data  bearing  upon 
this  point  are  available. 

It  would  be  a  matter  of  no  great  difficulty  to  develop  the  theory  of  ionic 
groups  in  much  greater  detail  than  has  here  been  attempted,  for  when 
uncomplicated  by  the  effects  of  irregular  distribution  and  diffusion  the 
theory  lends  itself  readily  to  analytical  treatment.  But  in  view  of  the 
small  amount  of  experimental  data  that  are  available  for  testing  the  con- 
clusions it  seems  inadvisable  to  carry  the  development  further  at  present. 

SUMMARY. 

In  the  case  of  a  homogeneous  substance  having  only  one  band  in  its 
phosphorescence  spectrum  and  uniformly  excited  throughout,  the  dis- 
sociation theory  first  suggested  by  Wiedemann  and  Schmidt  leads  to  a 
linear  relation  between  t  and  I~'  during  the  decay  of  phosphorescence. 
The  observed  decay  curve,  however — i.  e.,  the  curve  obtained  when  7~}  is 
plotted  as  a  function  of  t- — usually  shows  a  more  or  less  sharp  curvature  in 
the  early  stages  of  decay,  and  only  later  becomes  straight.  Using  the  dis- 
sociation theory  as  a  basis,  we  have  discussed  first  the  influence  upon  the 
decay  curve  of  various  disturbing  factors,  such  as  absorption  of  the  exciting 
and  emitted  light,  diffusion,  etc.  We  have  then  considered  the  effect  upon 
the  various  phenomena  of  phosphorescence  of  the  formation  of  complex  ions, 
which  are  assumed  to  be  produced  by  the  attachment  of  simple  ions  to 
neutral  molecules;  and  after  the  introduction  of  certain  hypotheses  regard- 
ing the  behavior  of  such  ionic  groups,  an  analytical  theory  is  developed. 

Briefly  the  results  of  the  discussion  are  as  follows: 

While  the  observations  thus  far  made  on  the  decay  of  phosphorescence 
are  perhaps  not  sufficient  to  definitely  disprove  Becquerel's  explanation  of 
the  form  of  the  decay  curve,  viz,  that  the  curvature  is  due  to  the  existence  of 
two  bands  in  the  phosphorescence  spectrum,  numerous  objections  to  this 
explanation  are  discussed.  For  example,  the  fact  that  the  decay  curve  has 
the  usual  form  even  when  precautions  are  taken  to  study  only  a  single  band ; 
that  the  phosphorescence  spectrum  remains  unaltered  during  decay;  the 
fact  that  the  decay  curve  is  of  the  same  type  for  all  substances  thus  far 
studied;  and  the  fact  that  Becquerel's  explanation  takes  no  account  of 
hysteresis  phenomena,  the  effect  of  infra-red  rays,  etc. 

2.  It  is  shown  that  lack  of  transparency  in  the  phosphorescent  sub- 
stance, resulting  in  the  absorption  of  either  the  exciting  light  or  the  emitted 
light  or  both,  will  cause  a  slight  curvature  in  the  decay  curve.     But  the 
effect  can  under  no  circumstances  be  sufficient  to  account  for  the  observed 
deviation  from  linearity. 

3.  Lack  of  uniformity  in  the  distribution  of  the  active  material  through- 
out the  mass  of  the  phosphorescent  substance  will  cause  a  deviation  from 
linearity  corresponding  to  that  actually  observed.     Under  certain  con- 


PHENOMENA    O£    PHOSPHORESCENCE.  223 

ditions  the  effect  is  the  same  as  though  the  substance  possessed  two  or 
more  bands,  coincident  as  regards  wave-length,  but  differing  in  rate  of 
decay.  It  is  pointed  out  that  such  a  lack  of  homogeneity  is  to  be  expected 
from  the  method  of  preparation  of  phosphorescent  substances,  and  has  a 
great  deal  to  do  with  the  complexity  of  the  phenomena. 

4.  Diffusion  of  the  ions  will  produce  effects  similar  to  those  caused  by 
irregularities  in  the  distribution  of  the  active  material  without  diffusion, 
but  the  effects  will  be  more  marked.     An  explanation  of  hysteresis  effects 
may  also  be  based  upon  the  presence  of  diffusion. 

5.  The  assumption  of  complex  or  grouped  ions  is  shown  to  lead  to  a  law 
of   decay   which   agrees   closely   with   experiment.     Ionic   grouping   also 
accounts  satisfactorily  for  the  dependence  of  the  decay  curve  upon  the 
duration  of  excitation  and  the  previous  history  of  the  substance,  and  for 
the  effect  of  exposure  to  the  infra-red. 

The  preceding  discussion  appears  to  justify  the  conclusion  that  the  dis- 
sociation theory  proposed  by  Wiedemann  and  Schmidt  accounts  satisfac- 
torily for  all  the  phenomena  of  phosphorescence  thus  far  studied.  Addi- 
tional quantitative  data  are  greatly  needed,  however,  in  order  to  make  pos- 
sible the  further  development  of  the  theory.  The  present  difficulty  is  not 
so  much  in  accounting  for  the  observed  facts  as  in  discriminating  between 
different  hypotheses  that  are  at  present  equally  plausible,  and  in  deciding  to 
what  extent  various  recognized  sources  of  disturbance  are  of  importance. 
The  form  of  the  decay  curve,  for  example,  may  be  accounted  for  equally  well 
by  the  assumption  of  two  bands,  by  an  irregular  distribution  of  the  active 
substance,  by  diffusion,  or  by  ionic  grouping,  and  is  unquestionably  modi- 
fied by  absorption.  No  quantitative  test  of  either  explanation  is  possible 
until  the  relative  importance  of  the  other  factors  is  known.  It  may  be,  for 
instance,  that  diffusion  does  not  occur  at  all;  and  it  is  possible,  but  not 
probable,  that  the  disturbances  due  to  lack  of  homogeneity  are  of  no  signifi- 
cance. Experiments  that  will  furnish  a  definite  answer  to  questions  of  this 
sort  are  of  the  greatest  importance  in  the  development  of  this  or  any  other 
theory  of  phosphorescence. 


INDEX. 


Absorption: 

Beer's  Law  of,  25,  33,  39. 
Coefficient  of,  30-33. 
Eosin,  189. 
Resorufin,  189. 
Fluorescence,  167-174. 

jEsculin,  fluorescence  and  transmission  spectra  of,  23. 
Andrews,  preparation  of  phosphorescent  compounds, 
110. 

Angstrom,  distribution  of  energy  in  flame  of  Hefner 

lamp,  178. 
Anthracene  vapor: 

Fluorescence  of,  163. 

Effect  of  fluorescence  on  conductivity  of,  163,  166. 

Balmain's  paint: 

Fluorescence  spectrum  of,  97. 
Phosphorescence  decay  of,  67,  68,  211. 
Effect  of— 

Delay  in  heating,  105,  106. 

Infra-red  rays,  81. 

Previous  history,  100. 

Temperature,  97-101. 

Time  of  excitation,  97,  101,  102,  103,  133, 

134. 

Hysteresis  effect  in,  63. 
Law  of,  202,  203. 

Thermo-luminescence  of,  102-107. 
Becquerel,  Ed.: 

Cause  of  change  in  phosphorescence  color,  49. 
Phosphorescence  decay,  51,  52,  53,  67,  202. 
Becquerel,  H.,  phosphorescence  decay,  51,  52,  53,  131, 

202,  203. 

Beer's  law,  25,  33,  39. 

Buckingham,  ionic  theory  of  fluorescence,  26,  39. 
Burke: 

Change  in  absorption  due  to  fluorescence,  147. 
Fluorescence  absorption,  167. 

Cadmium  salts: 

Fluorescence  of ,  112,  113,  114. 
Cadmium-sodium  compounds — 
Decay  curve  of,  121,  124. 
Phosphorescence  spectra  of,  120. 
CdSO4+xMnSOi: 

Kathodo-luminescence  of,  139. 
Luminescence  of,  by  ultra-violet  light  and  by 

Roentgen  rays,  139. 
Camichel: 

Conductivity  of  fluorescent  liquids,  151,  198. 
Fluorescence  absorption,  167. 
Canary  glass,  fluorescence  and  transmission  spectra  of, 

19. 
Chlorophyll,  fluorescence  and  transmission  spectra  of, 

17,  18. 
Coblentz,  W.  W.: 

Absorption  bands  in  spectrum  of  ZnS,  84. 
Energy  of  acetylene  spectrum,  175,  179. 
Infra-red  spectrum  of  chlorophyll,  17,  18. 
Conductivity  of  solutions,  effect  of  light  on,  147,  150, 

151,  198. 

Cunningham,  J.  A.,  effect  of  light  on  the  conductivity 
of  solutions,  147,  198. 

Dahms,  effect  of  infra-red  rays  on  phosphorescence, 

71,  77,  83. 

Darwin,  phosphorescence  decay,  51,  68. 
Decay  of  phosphorescence  (see  Phosphorescence). 
Distribution  of  energy: 

In  acetylene  flame,  125,  178,  179. 
In  Hefner  lamp  flame,  178. 
In  spectra  of — 
Eosin,  186. 
Fluorescein,  186. 
Resorufin,  186. 
Sidot  blende,  125-129. 
Distribution  of  intensities  in  spectra  of— 
Daylight,  14,  15. 
Eosin,  182. 
Fluorescein,  182. 
Resorufin,  182. 

224 


Elster  and  Geitel,  photo-electric  activity  of  lumines- 
cent substances,  198 

Kljton,  spectra  of  anthracene  vapor,  163. 
"Emanations-pulver": 

Decay  of  phosphorescence,  67. 

Effect  of  infra-red  rays,  75,  76,  77,  81. 
Energy,  distribution  of  (see  Distribution). 
Eosin: 

Coefficient  of  absorption,  190. 

Distribution  of  energy  in  spectrum,  186. 

Distribution  of  intensities  in  spectrum,  182. 

Fluorescence  spectra,  9. 

Fluorescence  vs.  exciting  power  of  various  wave- 
lengths, 192,  193. 

Photo-electric  action,  159,  161. 

Fluorescein: 

Distribution  of  energy  in  spectrum,  186. 
Distribution  of  intensities  in  spectrum,  182. 
Effect— 

Of  absorption,  6. 
Of  dilution,  7,  8. 
Spectra,  5,  6,  9. 
Photo-electric  action,  161. 
Fluorescence,  25. 

As  a  function  of  wave-length,  9. 
Absorption,  167-174. 
And  absorption  of — 

Uranium  glass,  19,  20. 
Chlorophyll,  17. 
Quinine  sulphate,  16. 
Effect  of  dilution  on,  26,  35,  36. 
Specific  exciting  power  for  various  wave-lengths, 

192-194. 

Spectrum,  5,  6,  9,  10,  12,  13,  15,  17,  19,  22,  23,  97. 
Effect  of— 

Solvent,  23. 
Temperature,  135. 

Distribution  of  energy  in,  125,  126,  127. 
Fluorite,  transmission  and  fluorescence  spectra,  21,  22. 

Gibbs,  R.  C.,  fluorescence  and  absorption  of  uranium 

glass,  20. 
Goldman: 

Effect  of  light  on  ionization  of  liquids,  151. 

Photo-electric  effect,  163,  198. 

Hagenbach: 

Stokes' s  law,  1. 

Fluorescence  of  chlorophyll,  17. 
Henry,  Ch.: 

Conduction  of  iodine  vapor  due  to  light,  163. 

Law  of  phosphorescence  decay,  51,  52. 
Hodge,  P.,  photo-active  cells  with  fluorescent  electro- 
lytes, 152,  162,  198. 

Houstoun,  R.  A.,  fluorescence  absorption,  167. 
Howe,  H.  E.,  relation  of  fluorescence  to  conductivity, 
163,  198. 

Infra-red  rays: 

Effect  in  suppressing  phosphorescence,  134,  135. 
Effect  on— 

Decay  of  phosphorescence,  72,  73,  77-82,  84. 
Phosphorescence  when  applied  during  excita- 
tion, 75. 

Intensity,  distribution  of  (see  Distribution). 
Ionization: 

Of  liquids  by  light  waves,  150,  151. 
Theory  of  luminescence,  26,  39,  121. 
Ives  and  Luckiesh: 

Effect  of  infra-red  rays  on  decay  of  phosphores- 
cence, 77,  78. 
Form  of  phosphorescence  spectrum,  204. 

Kathodo-luminescence: 

Dependence  of,  upon  current  and  potential  differ- 
ence, 143,  144. 

Equation  for  intensity  of,  144. 
Of  various  substances,  139,  144. 
Kester,  phosphoroscope,  118. 

Knoblauch,  O.,  effect  of  solvent  on  fluorescent  spec- 
trum, 23. 
Kundt,  effect  of  solvent  on  absorption  bands,  23. 


225 


Lamansky,  Stokes's  law,  1. 

Lambert's  law,  applicable  to  fluorescent  solutions ,  31, 

39. 
Lenard: 

Conducting  power  of  phosphorescent  sulphides,  198 . 

Decay  of  phosphorescent  sulphides,  124. 

Equation  for  intensity  of  kathodo-luminescence, 
144. 

Relation  between  /""*  and  time,  109. 
Lenard  and  Klatt,  long-time  decay  substances,  124, 
Lommel: 

Classification  of  fluorescent  substances,  3,  23. 

Effect  of  dilution  on  fluorescence,  26. 

Fluorescence  of  quinine  sulphate,  16. 

Law  of  luminescence,  196. 
Lubarsch,  Stokes's  law,  1. 
Luminescence: 

Kathodo,  137-144. 

Thermo,  87-95. 

Theory  of,  195-200. 

Micheli,  F.  J.,  decay  of  phosphorescence  in  Balmain's 
paint,  99. 

Naphthalin-roth: 

Fluorescence  of,  1. 
Fluorescence  spectra,  10. 

Phosphorescence: 

Apparatus  for  study  of  rapid  decay,  109,  120. 

Change  of  color  during  decay,  49. 

Decay  curves,  115,  116,  117. 

Distribution  of  energy  in  phosphorescence  spectra, 

125,  127. 
Effect  of— 

Heating,  86,  98,  99,  101,  102,  105,  106,  116.  117. 
Infra-red  rays,  62,  72-84,  118, 133,  134, 135. 
Intensity  of  excitation,  65-67. 
Previous  history,  100. 
Time  of  excitation,  60-64,  97,  101-105,  117. 
Hysteresis  effect,  61,  63,  70. 
Hysteresis  t's.  temperature,  222. 
Laws  of,  51-53,  202-204. 
Of  aniline  dyes,  122. 
Of  cadmium  compounds,  124. 
Of  manganese  compounds,  122,  123. 
Suppression  of,  71,  77.  134,  135. 
Theories  of,  57,  59,  202-218. 
Phosphorescent  compounds,  110,  111. 
Photo-electric  effect: 

Relation  to  fluorescence,  164. 
With  fluorescent  electrolytes,  152-162. 
Pierce,  C.  A.: 

Becquerel's  equation  of  decay  of  phosphorescence, 

131. 
Distribution  of  energy  in  spectrum  of  Sidot  blende, 

125-127. 

Form  of  phosphorescence  spectrum,  203. 
Phosphorescence  decay,  85-87,  202,  207,  211. 

Quinine  sulphate,  fluorescence  and  transmission  spectra 
of,  15. 

Regner,  effect  of  light  on  the  conductivity  of  solutions, 

147,  198. 
Resorcin-blau,  fluorescence  and  transmission  spectra 

of,  13. 
Resorufin: 

Absorption  of,  167-174. 

Coefficient  of  absorption  of,  30,  31,  190. 

Effect  of  thickness  on  the  fluorescence  of,  37,  38. 

Distribution  of  energy  in  fluorescence  spectrum  of, 

186. 
Intensity  of  fluorescence  at  various  concentrations, 

35,  36. 
Specific  exciting  power  of  various  wave-lengths  on, 

193. 

Transmission  spectrum  of,  28. 
Rhodamin: 

Photo-electric  action  of,  161. 
Fluorescence  spectrum  of,  12. 
Rutherford: 

Kathodo-luminescence,  144. 

Law  of  recombination  of  ions,  202. 


Sidot  blende: 

Distribution  of  energy  in  spectra,  125-127. 
Luminescence — 

Due  to  Roentgen  rays,  41,  43,  143. 
Due  to  ultra-violet  light,  42,  44. 
Kathodo,  143-147. 
Intensity  of  vs.  voltage,  143. 
Thermo,  87-96. 
Phosphorescence — 

Decay  of,  47,  43,  53-57. 
Effect  of— 

Infra-red  rays,  74,  75,  78-81,  118. 
Various  temperatures,  86,  128,  129. 
Time  of  excitation,  135. 
Spectrum — 

Change  of  color,  49. 
Effect  of  temperature  on,  135. 
Intensity  of  color,  45,  46. 
Sphalerite,  absorption  bands  of,  84. 
Stark,  ionization  theory  of  fluorescence,  163. 
Stark  and  Steubing,  ionization  theory  of  fluorescence, 

164. 
Stenger: 

Effect  of  solvent  on  fluorescence  spectrum,  23. 
Stokes's  law,  1. 

Stewart,  G.  W.,  energy  of  spectrum  of  acetylene,  175. 
Stokes,  fluorescence  spectrum  of  chlorophyll,  17. 
Stokes's  law,  1,  11,  13,  16,  20.  24,  45,  196,  197,  201. 

Thermo-luminescence: 

Effect  of  delay  in  heating,  93,  94. 

In  Sidot  blende,  85-96. 

Saturation  effect,  92. 
Thompson,  J.  J.,  influence  of  light  on  conductivity  of 

Na  vapor,  165. 
Trowbridge,  C.  C.: 

Decay  of  phosphorescence  in  gases,  202. 

Ionic  theory  of  phosphorescence,  200. 
Tufts,  luminosity  curve,  185. 

Uranium  glass,  fluorescence  absorption  of,  167. 

Vogel,  distribution  of  intensities  in  spectrum  of  day- 
light, 14. 

Waggoner,  Dr.  C.  W.: 

Decay  of  phosphorescence  of  Sidot  blende,  136. 

Of  willemite,  211. 

Form  of  phosphorescence  spectrum,  204. 

Phosphorescence  of  short  duration,  109. 
Walter,  B.: 

Coefficient  of  absorption,  33. 

Effect  of  dilution  on  fluorescence,  26. 
Wehnelt,  kathodo-luminescence,  144. 
Werner,  A.: 

Form  of  phosphorescence  spectrum,  204. 

Luminescence  fatigue,  212. 
Wesendonck,  Stokes's  law,  1. 
Wick,  Miss  F.  G.: 

Absorption  of  resorufin,  25,  167,  190. 

Fluorescence  of  resorufin,  25. 
Wiedemann: 

Phosphorescence  decay  of  Balmain's  paint,  63. 

Law  of  phosphorescence  decay,  51. 

Theory  of  luminescence,  195. 
Wiedemann  and  Schmidt: 

Effect  of  infra-red  rays  on  phosphorescence,  213. 

Ionic  theory  of  luminescence,  123,  163,  195. 

Phosphorescent  effect  of  zinc  salts  as  impurities, 
113. 

Theory  of  decay  of  phosphorescence,  57,  59,  202. 
Wilkinson,  analysis  of  Cd  compounds  used  in  experi- 
ments, 112. 
Willemite: 

Decay  of  phosphorescence,  67,  211. 

Luminescence  of,  140,  141. 
Wood,  R.  W.,  fluorescence  absorption,  167,  172. 

Zeller,  C.: 

Form  of  phosphorescence  spectrum,  204. 
Phosphorescence  of  aniline  dyes,  manganese  com- 
pounds and  cadmium  compounds,  122- 
124. 
Zinc  sulphide: 

Law  of  decay  of  phosphorescence  of,  202,  203. 
(See    also     "Emanatioas-pulver"     and     "Sidot 
blende.") 


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